diff --git a/msat.mlpack b/msat.mlpack
index ad1bdbc5..c68f4682 100644
--- a/msat.mlpack
+++ b/msat.mlpack
@@ -5,6 +5,8 @@ Sat
 Solver
 Solver_types
 Theory_intf
+Tseitin
+Tseitin_intf
 
 #Arith
 #Cc
diff --git a/sat/formula_intf.ml b/sat/formula_intf.ml
index 0dbcd066..ac55cf23 100644
--- a/sat/formula_intf.ml
+++ b/sat/formula_intf.ml
@@ -22,6 +22,9 @@ module type S = sig
   val dummy : t
   (** Formula constants. A valid formula should never be physically equal to [dummy] *)
 
+  val fresh : unit -> t
+  (** Returns a fresh litteral, distinct from any other literal (used in cnf conversion) *)
+
   val neg : t -> t
   (** Formula negation *)
 
diff --git a/sat/sat.ml b/sat/sat.ml
index 6e828331..b1b29bba 100644
--- a/sat/sat.ml
+++ b/sat/sat.ml
@@ -45,11 +45,15 @@ module Fsat = struct
     in
     create, iter
 
+  let fresh = create
+
   let print fmt a =
     Format.fprintf fmt "%s%d" (if a < 0 then "~" else "") (abs a)
 
 end
 
+module Tseitin = Tseitin.Make(Fsat)
+
 module Stypes = Solver_types.Make(Fsat)
 
 module Exp = Explanation.Make(Stypes)
diff --git a/sat/sat.mli b/sat/sat.mli
index 2f7b61d3..06fc967e 100644
--- a/sat/sat.mli
+++ b/sat/sat.mli
@@ -1,12 +1,16 @@
 (* Copyright 2014 Guillaume Bury *)
 
+module Fsat : Formula_intf.S
+
+module Tseitin : Tseitin.S with type atom = Fsat.t
+
 module Make(Dummy: sig end) : sig
   (** Fonctor to make a pure SAT Solver module with built-in literals. *)
 
   exception Bad_atom
   (** Exception raised when a problem with atomic formula encoding is detected. *)
 
-  type atom = private int
+  type atom = Fsat.t
   (** Type for atoms, i.e boolean literals. *)
 
   type proof
diff --git a/sat/tseitin.ml b/sat/tseitin.ml
new file mode 100644
index 00000000..ef6acdf5
--- /dev/null
+++ b/sat/tseitin.ml
@@ -0,0 +1,329 @@
+(**************************************************************************)
+(*                                                                        *)
+(*                          Alt-Ergo Zero                                 *)
+(*                                                                        *)
+(*                  Sylvain Conchon and Alain Mebsout                     *)
+(*                      Universite Paris-Sud 11                           *)
+(*                                                                        *)
+(*  Copyright 2011. This file is distributed under the terms of the       *)
+(*  Apache Software License version 2.0                                   *)
+(*                                                                        *)
+(**************************************************************************)
+
+module type S = Tseitin_intf.S
+
+module Make (F : Formula_intf.S) = struct
+
+  exception Empty_Or
+  type combinator = And | Or | Imp | Not
+
+  type atom = F.t
+  type t =
+    | Lit of atom
+    | Comb of combinator * t list
+
+  let rec print fmt phi =
+    match phi with
+    | Lit a -> F.print fmt a
+    | Comb (Not, [f]) ->
+      Format.fprintf fmt "not (%a)" print f
+    | Comb (And, l) -> Format.fprintf fmt "(%a)" (print_list "and") l
+    | Comb (Or, l) ->  Format.fprintf fmt "(%a)" (print_list "or") l
+    | Comb (Imp, [f1; f2]) ->
+      Format.fprintf fmt "(%a => %a)" print f1 print f2
+    | _ -> assert false
+  and print_list sep fmt = function
+    | [] -> ()
+    | [f] -> print fmt f
+    | f::l -> Format.fprintf fmt "%a %s %a" print f sep (print_list sep) l
+
+  let make comb l = Comb (comb, l)
+
+  let value env c =
+    if List.mem c env then Some true
+    else if List.mem (make Not [c]) env then Some false
+    else None
+
+  (*
+  let rec lift_ite env op l =
+    try
+      let left, (c, t1, t2), right = Term.first_ite l in
+      begin
+        match value env c with
+        | Some true ->
+          lift_ite (c::env) op (left@(t1::right))
+        | Some false ->
+          lift_ite ((make Not [c])::env) op (left@(t2::right))
+        | None ->
+          Comb
+            (And,
+             [Comb
+                (Imp, [c; lift_ite (c::env) op (left@(t1::right))]);
+              Comb (Imp,
+                    [(make Not [c]);
+                     lift_ite
+                       ((make Not [c])::env) op (left@(t2::right))])])
+      end
+    with Not_found ->
+      begin
+        let lit =
+          match op, l with
+          | Eq, [Term.T t1; Term.T t2] ->
+            Literal.Eq (t1, t2)
+          | Neq, ts ->
+            let ts =
+              List.rev_map (function Term.T x -> x | _ -> assert false) ts in
+            Literal.Distinct (false, ts)
+          | Le, [Term.T t1; Term.T t2] ->
+            Literal.Builtin (true, Hstring.make "<=", [t1; t2])
+          | Lt, [Term.T t1; Term.T t2] ->
+            Literal.Builtin (true, Hstring.make "<", [t1; t2])
+          | _ -> assert false
+        in
+        Lit (F.make lit)
+      end
+
+  let make_lit op l = lift_ite [] op l
+  *)
+
+  let make_atom p = Lit p
+
+  let make_not f = make Not [f]
+  let make_and l = make And l
+  let make_imply f1 f2 = make Imp [f1; f2]
+  let make_equiv f1 f2 = make And [ make_imply f1 f2; make_imply f2 f1]
+  let make_xor f1 f2 = make Or [ make And [ make Not [f1]; f2 ];
+                                 make And [ f1; make Not [f2] ] ]
+
+  (* simplify formula *)
+  let rec sform = function
+    | Comb (Not, [Lit a]) -> Lit (F.neg a)
+    | Comb (Not, [Comb (Not, [f])]) -> sform f
+    | Comb (Not, [Comb (Or, l)]) ->
+      let nl = List.rev_map (fun a -> sform (Comb (Not, [a]))) l in
+      Comb (And, nl)
+    | Comb (Not, [Comb (And, l)]) ->
+      let nl = List.rev_map (fun a -> sform (Comb (Not, [a]))) l in
+      Comb (Or, nl)
+    | Comb (Not, [Comb (Imp, [f1; f2])]) ->
+      Comb (And, [sform f1; sform (Comb (Not, [f2]))])
+    | Comb (And, l) ->
+      Comb (And, List.rev_map sform l)
+    | Comb (Or, l) ->
+      Comb (Or, List.rev_map sform l)
+    | Comb (Imp, [f1; f2]) ->
+      Comb (Or, [sform (Comb (Not, [f1])); sform f2])
+    | Comb ((Imp | Not), _) -> assert false
+    | Lit _ as f -> f
+
+
+  let ( @@ ) l1 l2 = List.rev_append l1 l2
+  let ( @ ) = `Use_rev_append_instead   (* prevent use of non-tailrec append *)
+
+  let make_or = function
+    | [] -> raise Empty_Or
+    | [a] -> a
+    | l -> Comb (Or, l)
+
+  let distrib l_and l_or =
+    let l =
+      if l_or = [] then l_and
+      else
+        List.rev_map
+          (fun x ->
+             match x with
+             | Lit _ -> Comb (Or, x::l_or)
+             | Comb (Or, l) -> Comb (Or, l @@ l_or)
+             | _ -> assert false
+          ) l_and
+    in
+    Comb (And, l)
+
+  let rec flatten_or = function
+    | [] -> []
+    | Comb (Or, l)::r -> l @@ (flatten_or r)
+    | Lit a :: r -> (Lit a)::(flatten_or r)
+    | _ -> assert false
+
+  let rec flatten_and = function
+    | [] -> []
+    | Comb (And, l)::r -> l @@ (flatten_and r)
+    | a :: r -> a::(flatten_and r)
+
+
+  let rec cnf f =
+    match f with
+    | Comb (Or, l) ->
+      begin
+        let l = List.rev_map cnf l in
+        let l_and, l_or =
+          List.partition (function Comb(And,_) -> true | _ -> false) l in
+        match l_and with
+        | [ Comb(And, l_conj) ] ->
+          let u = flatten_or l_or in
+          distrib l_conj u
+
+        | Comb(And, l_conj) :: r ->
+          let u = flatten_or l_or in
+          cnf (Comb(Or, (distrib l_conj u)::r))
+
+        | _ ->
+          begin
+            match flatten_or l_or with
+            | [] -> assert false
+            | [r] -> r
+            | v -> Comb (Or, v)
+          end
+      end
+    | Comb (And, l) ->
+      Comb (And, List.rev_map cnf l)
+    | f -> f
+
+  let rec mk_cnf = function
+    | Comb (And, l) ->
+      List.fold_left (fun acc f ->  (mk_cnf f) @@ acc) [] l
+
+    | Comb (Or, [f1;f2]) ->
+      let ll1 = mk_cnf f1 in
+      let ll2 = mk_cnf f2 in
+      List.fold_left
+        (fun acc l1 -> (List.rev_map (fun l2 -> l1 @@ l2)ll2) @@ acc) [] ll1
+
+    | Comb (Or, f1 :: l) ->
+      let ll1 = mk_cnf f1 in
+      let ll2 = mk_cnf (Comb (Or, l)) in
+      List.fold_left
+        (fun acc l1 -> (List.rev_map (fun l2 -> l1 @@ l2)ll2) @@ acc) [] ll1
+
+    | Lit a -> [[a]]
+    | Comb (Not, [Lit a]) -> [[F.neg a]]
+    | _ -> assert false
+
+
+  let rec unfold mono f =
+    match f with
+    | Lit a -> a::mono
+    | Comb (Not, [Lit a]) ->
+      (F.neg a)::mono
+    | Comb (Or, l) ->
+      List.fold_left unfold mono l
+    | _ -> assert false
+
+  let rec init monos f =
+    match f with
+    | Comb (And, l) ->
+      List.fold_left init monos l
+    | f -> (unfold [] f)::monos
+
+  let make_cnf f =
+    let sfnc = cnf (sform f) in
+    init [] sfnc
+
+  let mk_proxy = F.fresh
+      (*
+    let cpt = ref 0 in
+    fun () ->
+      let t = AETerm.make
+          (Symbols.name (Hstring.make ("PROXY__"^(string_of_int !cpt))))
+          [] Ty.Tbool
+      in
+      incr cpt;
+      F.make (Literal.Eq (t, AETerm.true_))
+      *)
+
+  let acc_or = ref []
+  let acc_and = ref []
+
+  (* build a clause by flattening (if sub-formulas have the
+     same combinator) and proxy-ing sub-formulas that have the
+     opposite operator. *)
+  let rec cnf f = match f with
+    | Lit a -> None, [a]
+    | Comb (Not, [Lit a]) -> None, [F.neg a]
+
+    | Comb (And, l) ->
+      List.fold_left
+        (fun (_, acc) f ->
+           match cnf f with
+           | _, [] -> assert false
+           | cmb, [a] -> Some And, a :: acc
+           | Some And, l ->
+             Some And, l @@ acc
+           (* let proxy = mk_proxy () in *)
+           (* acc_and := (proxy, l) :: !acc_and; *)
+           (* proxy :: acc *)
+           | Some Or, l ->
+             let proxy = mk_proxy () in
+             acc_or := (proxy, l) :: !acc_or;
+             Some And, proxy :: acc
+           | None, l -> Some And, l @@ acc
+           | _ -> assert false
+        ) (None, []) l
+
+    | Comb (Or, l) ->
+      List.fold_left
+        (fun (_, acc) f ->
+           match cnf f with
+           | _, [] -> assert false
+           | cmb, [a] -> Some Or, a :: acc
+           | Some Or, l ->
+             Some Or, l @@ acc
+           (* let proxy = mk_proxy () in *)
+           (* acc_or := (proxy, l) :: !acc_or; *)
+           (* proxy :: acc *)
+           | Some And, l ->
+             let proxy = mk_proxy () in
+             acc_and := (proxy, l) :: !acc_and;
+             Some Or, proxy :: acc
+           | None, l -> Some Or, l @@ acc
+           | _ -> assert false
+        ) (None, []) l
+
+    | _ -> assert false
+
+  let cnf f =
+    let acc = match f with
+      | Comb (And, l) -> List.rev_map (fun f -> snd(cnf f)) l
+      | _ -> [snd (cnf f)]
+    in
+    let proxies = ref [] in
+    (* encore clauses that make proxies in !acc_and equivalent to
+       their clause *)
+    let acc =
+      List.fold_left
+        (fun acc (p,l) ->
+           proxies := p :: !proxies;
+           let np = F.neg p in
+           (* build clause [cl = l1 & l2 & ... & ln => p] where [l = [l1;l2;..]]
+              also add clauses [p => l1], [p => l2], etc. *)
+           let cl, acc =
+             List.fold_left
+               (fun (cl,acc) a -> (F.neg a :: cl), [np; a] :: acc)
+               ([p],acc) l in
+           cl :: acc
+        ) acc !acc_and
+    in
+    (* encore clauses that make proxies in !acc_or equivalent to
+       their clause *)
+    let acc =
+      List.fold_left
+        (fun acc (p,l) ->
+           proxies := p :: !proxies;
+           (* add clause [p => l1 | l2 | ... | ln], and add clauses
+              [l1 => p], [l2 => p], etc. *)
+           let acc = List.fold_left (fun acc a -> [p; F.neg a]::acc)
+               acc l in
+           (F.neg p :: l) :: acc
+        ) acc !acc_or
+    in
+    acc
+
+  let make_cnf f =
+    acc_or := [];
+    acc_and := [];
+    cnf (sform f)
+
+  (* Naive CNF XXX remove???
+     let make_cnf f = mk_cnf (sform f)
+  *)
+end
diff --git a/sat/tseitin.mli b/sat/tseitin.mli
new file mode 100644
index 00000000..1eb9dc6c
--- /dev/null
+++ b/sat/tseitin.mli
@@ -0,0 +1,9 @@
+(*
+MSAT is free software, using the Apache license, see file LICENSE
+Copyright 2014 Guillaume Bury
+Copyright 2014 Simon Cruanes
+*)
+
+module type S = Tseitin_intf.S
+
+module Make : functor (F : Formula_intf.S) -> S with type atom = F.t
diff --git a/sat/tseitin_intf.ml b/sat/tseitin_intf.ml
new file mode 100644
index 00000000..6f6f8986
--- /dev/null
+++ b/sat/tseitin_intf.ml
@@ -0,0 +1,38 @@
+(**************************************************************************)
+(*                                                                        *)
+(*                          Alt-Ergo Zero                                 *)
+(*                                                                        *)
+(*                  Sylvain Conchon and Alain Mebsout                     *)
+(*                      Universite Paris-Sud 11                           *)
+(*                                                                        *)
+(*  Copyright 2011. This file is distributed under the terms of the       *)
+(*  Apache Software License version 2.0                                   *)
+(*                                                                        *)
+(**************************************************************************)
+
+module type S = sig
+
+  (** The type of ground formulas *)
+  type t
+  type atom
+
+  val make_atom : atom -> t
+  (** [make_pred p] builds the atomic formula [p = true].
+      @param sign the polarity of the atomic formula *)
+
+  val make_not : t -> t
+  val make_and : t list -> t
+  val make_or : t list -> t
+  val make_imply : t -> t -> t
+  val make_equiv : t -> t -> t
+  val make_xor : t -> t -> t
+
+  val make_cnf : t -> atom list list
+  (** [make_cnf f] returns a conjunctive normal form of [f] under the form: a
+      list (which is a conjunction) of lists (which are disjunctions) of
+      literals. *)
+
+  val print : Format.formatter -> t -> unit
+  (** [print fmt f] prints the formula on the formatter [fmt].*)
+
+end