diff --git a/src/dune_project.ml b/src/dune_project.ml
index 52b149a0..31878120 100644
--- a/src/dune_project.ml
+++ b/src/dune_project.ml
@@ -149,8 +149,7 @@ module Lang = struct
     in
     let ver =
       Sexp.Of_sexp.parse Syntax.Version.t Univ_map.empty
-        (Atom (ver_loc, Sexp.Atom.of_string ver))
-    in
+        (Atom (ver_loc, Sexp.Atom.of_string_exn Sexp.Atom.Dune ver)) in
     match Hashtbl.find langs name with
     | None ->
       Loc.fail name_loc "Unknown language %S.%s" name
diff --git a/src/loc.ml b/src/loc.ml
index 28d7a1dc..f1b78fba 100644
--- a/src/loc.ml
+++ b/src/loc.ml
@@ -11,7 +11,8 @@ let int n = Usexp.Atom (Usexp.Atom.of_int n)
 let string = Usexp.atom_or_quoted_string
 let record l =
   let open Usexp in
-  List (List.map l ~f:(fun (n, v) -> List [Atom(Atom.of_string n); v]))
+  List (List.map l ~f:(fun (n, v) ->
+    List [Atom(Atom.of_string_exn Atom.Dune n); v]))
 
 let sexp_of_position_no_file (p : Lexing.position) =
   record
diff --git a/src/stdune/sexp.ml b/src/stdune/sexp.ml
index 69dad943..8e8e96a6 100644
--- a/src/stdune/sexp.ml
+++ b/src/stdune/sexp.ml
@@ -34,7 +34,8 @@ module To_sexp = struct
   let string_set set = list atom (String.Set.to_list set)
   let string_map f map = list (pair atom f) (String.Map.to_list map)
   let record l =
-    List (List.map l ~f:(fun (n, v) -> List [Atom(Atom.of_string n); v]))
+    List (List.map l ~f:(fun (n, v) ->
+      List [Atom(Atom.of_string_exn Usexp.Atom.Dune n); v]))
   let string_hashtbl f h =
     string_map f
       (Hashtbl.foldi h ~init:String.Map.empty ~f:(fun key data acc ->
@@ -54,7 +55,7 @@ module To_sexp = struct
 
   let record_fields (l : field list) =
     List (List.filter_map l ~f:(fun (k, v) ->
-      Option.map v ~f:(fun v -> List[Atom (Atom.of_string k); v])))
+      Option.map v ~f:(fun v -> List[Atom (Atom.of_string_exn Atom.Dune k); v])))
 
   let unknown _ = unsafe_atom_of_string "<unknown>"
 end
@@ -296,13 +297,13 @@ module Of_sexp = struct
   let string = plain_string (fun ~loc:_ x -> x)
   let int =
     basic "Integer" (fun s ->
-      match int_of_string s with
+      match int_of_string (s Atom.Dune) with
       | x -> Ok x
       | exception _ -> Error ())
 
   let float =
     basic "Float" (fun s ->
-      match float_of_string s with
+      match float_of_string (s Atom.Dune) with
       | x -> Ok x
       | exception _ -> Error ())
 
diff --git a/src/usexp/atom.ml b/src/usexp/atom.ml
new file mode 100644
index 00000000..471301e1
--- /dev/null
+++ b/src/usexp/atom.ml
@@ -0,0 +1,55 @@
+type t = A of string [@@unboxed]
+
+let invalid_argf fmt = Printf.ksprintf invalid_arg fmt
+
+type syntax = Jbuild | Dune
+
+let string_of_syntax = function
+  | Jbuild -> "jbuild"
+  | Dune -> "dune"
+
+let (is_valid_jbuild, is_valid_dune) =
+  let rec jbuild s i len =
+    i = len ||
+    match String.unsafe_get s i with
+    | '"' | '(' | ')' | ';' | '\000'..'\032' | '\127'..'\255' -> false
+    | _ -> jbuild s (i + 1) len
+  in
+  let rec dune s i len =
+    i = len ||
+    match String.unsafe_get s i with
+    | '%' | '"' | '(' | ')' | ';' | '\000'..'\032' | '\127'..'\255' -> false
+    | _ -> dune s (i + 1) len
+  in
+  let make looper s =
+    let len = String.length s in
+    len > 0 && looper s 0 len
+  in
+  (make jbuild, make dune)
+
+let of_string syn s =
+  match syn with
+  | Jbuild when is_valid_jbuild s -> Some (A s)
+  | Dune when is_valid_dune s -> Some (A s)
+  | _ -> None
+
+let of_string_exn syn s =
+  match of_string syn s with
+  | Some s -> s
+  | None ->
+    invalid_argf "'%s' is not a valid %s atom" s (string_of_syntax syn)
+
+let to_string (A t) syntax =
+  match syntax with
+  | Jbuild -> t
+  | Dune ->
+    if is_valid_dune t then
+      t
+    else
+      invalid_argf "Jbuild atom '%s' is not a valid dune atom" t
+
+let of_int i = of_string_exn Dune (string_of_int i)
+let of_float x = of_string_exn Dune (string_of_float x)
+let of_bool x = of_string_exn Dune (string_of_bool x)
+let of_int64 i = of_string_exn Dune (Int64.to_string i)
+let of_digest d = of_string_exn Dune (Digest.to_hex d)
diff --git a/src/usexp/atom.mli b/src/usexp/atom.mli
new file mode 100644
index 00000000..40bd6658
--- /dev/null
+++ b/src/usexp/atom.mli
@@ -0,0 +1,15 @@
+type t = private A of string [@@unboxed]
+
+type syntax = Jbuild | Dune
+
+val of_string : syntax -> string -> t option
+
+val of_string_exn : syntax -> string -> t
+
+val to_string : t -> syntax -> string
+
+val of_int : int -> t
+val of_float : float -> t
+val of_bool : bool -> t
+val of_int64 : Int64.t -> t
+val of_digest : Digest.t -> t
diff --git a/src/usexp/lexer.mli b/src/usexp/lexer.mli
index 5015e1ad..22407e45 100644
--- a/src/usexp/lexer.mli
+++ b/src/usexp/lexer.mli
@@ -1,7 +1,3 @@
-module Atom : sig
-  type t = A of string [@@unboxed]
-end
-
 module Token : sig
   type t =
     | Atom          of Atom.t
diff --git a/src/usexp/lexer.mll b/src/usexp/lexer.mll
index 69912d31..ef072f06 100644
--- a/src/usexp/lexer.mll
+++ b/src/usexp/lexer.mll
@@ -1,8 +1,4 @@
 {
-module Atom = struct
-  type t = A of string [@@unboxed]
-end
-
 module Token = struct
   type t =
     | Atom          of Atom.t
@@ -128,7 +124,7 @@ and jbuild_atom acc start = parse
         error lexbuf "Internal error in the S-expression parser, \
                       please report upstream.";
       lexbuf.lex_start_p <- start;
-      Token.Atom (A acc)
+      Token.Atom (Atom.of_string_exn Jbuild acc)
     }
 
 and quoted_string mode = parse
@@ -248,7 +244,7 @@ and token = parse
       Quoted_string s
     }
   | atom_char_dune+ as s
-    { Token.Atom (A s) }
+    { Token.Atom (Atom.of_string_exn Dune s) }
   | eof
     { Eof }
 
diff --git a/src/usexp/usexp.ml b/src/usexp/usexp.ml
index 48a3dd32..0252bc66 100644
--- a/src/usexp/usexp.ml
+++ b/src/usexp/usexp.ml
@@ -10,34 +10,7 @@ module Bytes = struct
     UnlabeledBytes.blit_string src src_pos dst dst_pos len
 end
 
-module Atom = struct
- type t = Lexer.Atom.t = A of string [@@unboxed]
-
- let is_valid =
-   let rec loop s i len =
-     i = len ||
-     match String.unsafe_get s i with
-     | '"' | '(' | ')' | ';' | '\000'..'\032' | '\127'..'\255' -> false
-     | _ -> loop s (i + 1) len
-   in
-   fun s ->
-     let len = String.length s in
-     len > 0 && loop s 0 len
-
- (* XXX eventually we want to report a nice error message to the user
-     at the point the conversion is made. *)
-  let of_string s =
-    if is_valid s then A s
-    else invalid_arg(Printf.sprintf "Usexp.Atom.of_string: %S" s)
-
-  let of_int i = A (string_of_int i)
-  let of_float x = A (string_of_float x)
-  let of_bool x = A (string_of_bool x)
-  let of_int64 i = A (Int64.to_string i)
-  let of_digest d = A (Digest.to_hex d)
-
-  let to_string (A s) = s
-end
+module Atom = Atom
 
 type t =
   | Atom of Atom.t
@@ -46,15 +19,14 @@ type t =
 
 type sexp = t
 
-let atom s =
-  if Atom.is_valid s then Atom (A s)
-  else invalid_arg "Usexp.atom"
+let atom s = Atom (Atom.of_string_exn Dune s)
 
-let unsafe_atom_of_string s = Atom(A s)
+let unsafe_atom_of_string s = atom s
 
 let atom_or_quoted_string s =
-  if Atom.is_valid s then Atom (A s)
-  else Quoted_string s
+  match Atom.of_string Atom.Dune s with
+  | None -> Quoted_string s
+  | Some a -> Atom a
 
 let quote_length s =
   let n = ref 0 in
@@ -117,13 +89,13 @@ let quoted s =
   Bytes.unsafe_to_string s'
 
 let rec to_string = function
-  | Atom (A s) -> s
+  | Atom a -> Atom.to_string a Atom.Dune
   | Quoted_string s -> quoted s
   | List l -> Printf.sprintf "(%s)" (List.map l ~f:to_string |> String.concat ~sep:" ")
 
 let rec pp ppf = function
-  | Atom (A s) ->
-    Format.pp_print_string ppf s
+  | Atom s ->
+    Format.pp_print_string ppf (Atom.to_string s Atom.Dune)
   | Quoted_string s ->
     Format.pp_print_string ppf (quoted s)
   | List [] ->
@@ -164,7 +136,7 @@ let pp_print_quoted_string ppf s =
     Format.pp_print_string ppf (quoted s)
 
 let rec pp_split_strings ppf = function
-  | Atom (A s) -> Format.pp_print_string ppf s
+  | Atom s -> Format.pp_print_string ppf (Atom.to_string s Atom.Dune)
   | Quoted_string s -> pp_print_quoted_string ppf s
   | List [] ->
     Format.pp_print_string ppf "()"
@@ -249,8 +221,9 @@ module Ast = struct
     | List of Loc.t * t list
 
   let atom_or_quoted_string loc s =
-    if Atom.is_valid s then Atom (loc, A s)
-    else Quoted_string (loc, s)
+    match Atom.of_string Atom.Dune s with
+    | None -> Quoted_string (loc, s)
+    | Some a -> Atom (loc, a)
 
   let loc (Atom (loc, _) | Quoted_string (loc, _) | List (loc, _)) = loc
 
diff --git a/src/usexp/usexp.mli b/src/usexp/usexp.mli
index 6ade200b..781625e0 100644
--- a/src/usexp/usexp.mli
+++ b/src/usexp/usexp.mli
@@ -7,20 +7,19 @@ module Atom : sig
   (** Acceptable atoms are composed of chars in the range ['!' .. '~'] excluding
       [' ' '"' '(' ')' ';' '\\'], and must be nonempty. *)
 
-  val is_valid : string -> bool
-  (** [is_valid s] checks that [s] respects the constraints to be an atom. *)
+  type syntax = Jbuild | Dune
 
- val of_string : string -> t
-  (** Convert a string to an atom.  If the string contains invalid
-     characters, raise [Invalid_argument]. *)
+ val of_string : syntax -> string -> t option
+
+ val of_string_exn : syntax -> string -> t
+
+  val to_string : t -> syntax -> string
 
   val of_int : int -> t
   val of_float : float -> t
   val of_bool : bool -> t
   val of_int64 : Int64.t -> t
   val of_digest : Digest.t -> t
-
-  val to_string : t -> string
 end
 
 module Loc : sig
diff --git a/test/unit-tests/sexp.mlt b/test/unit-tests/sexp.mlt
index 67e516e1..821f4488 100644
--- a/test/unit-tests/sexp.mlt
+++ b/test/unit-tests/sexp.mlt
@@ -140,7 +140,7 @@ parse {|"$bar%foo%"|}
 
 parse {|\%{foo}|}
 [%%expect{|
-- : parse_result = Same (Ok [\%{foo}])
+Exception: Invalid_argument "'\\%{foo}' is not a valid dune atom".
 |}]
 
 parse {|\${foo}|}
@@ -150,15 +150,15 @@ parse {|\${foo}|}
 
 parse {|\$bar%foo%|}
 [%%expect{|
-- : parse_result = Same (Ok [\$bar%foo%])
+Exception: Invalid_argument "'\\$bar%foo%' is not a valid dune atom".
 |}]
 
 parse {|\$bar\%foo%|}
 [%%expect{|
-- : parse_result = Same (Ok [\$bar\%foo%])
+Exception: Invalid_argument "'\\$bar\\%foo%' is not a valid dune atom".
 |}]
 
 parse {|\$bar\%foo%{bar}|}
 [%%expect{|
-- : parse_result = Same (Ok [\$bar\%foo%{bar}])
+Exception: Invalid_argument "'\\$bar\\%foo%{bar}' is not a valid dune atom".
 |}]
diff --git a/test/unit-tests/sexp_tests.ml b/test/unit-tests/sexp_tests.ml
index 4fd2bc3a..2d4e3fbf 100644
--- a/test/unit-tests/sexp_tests.ml
+++ b/test/unit-tests/sexp_tests.ml
@@ -23,13 +23,15 @@ let () =
         | Atom _ -> true
         | _      -> false
       in
-      if Usexp.Atom.is_valid s <> parser_recognizes_as_atom then begin
+      let valid_dune_atom =
+        Option.is_some (Usexp.Atom.of_string Dune s) in
+      if valid_dune_atom <> parser_recognizes_as_atom then begin
         Printf.eprintf
           "Usexp.Atom.is_valid error:\n\
            - s = %S\n\
            - Usexp.Atom.is_valid s = %B\n\
            - parser_recognizes_as_atom = %B\n"
-          s (Usexp.Atom.is_valid s) parser_recognizes_as_atom;
+          s valid_dune_atom parser_recognizes_as_atom;
         exit 1
       end;
       if printed_as_atom && not parser_recognizes_as_atom then begin