diff --git a/src/action.ml b/src/action.ml
index 153a700b..d6b25de3 100644
--- a/src/action.ml
+++ b/src/action.ml
@@ -697,7 +697,7 @@ end
 type exec_context =
   { context : Context.t option
   ; purpose : Process.purpose
-  ; env     : string array
+  ; env     : Env.t
   }
 
 let exec_run_direct ~ectx ~dir ~env_extra ~stdout_to ~stderr_to prog args =
@@ -715,8 +715,9 @@ let exec_run_direct ~ectx ~dir ~env_extra ~stdout_to ~stderr_to prog args =
      invalid_prefix ("_build/" ^ target.name);
      invalid_prefix ("_build/install/" ^ target.name);
   end;
-  let env = Env.extend_env ~vars:env_extra ~env:ectx.env in
-  Process.run Strict ~dir:(Path.to_string dir) ~env ~stdout_to ~stderr_to
+  let env = Env.extend ectx.env ~vars:env_extra in
+  Process.run Strict ~dir:(Path.to_string dir) ~env:(Env.to_unix env)
+    ~stdout_to ~stderr_to
     ~purpose:ectx.purpose
     (Path.reach_for_running ~from:dir prog) args
 
@@ -892,7 +893,7 @@ and exec_list l ~ectx ~dir ~env_extra ~stdout_to ~stderr_to =
 let exec ~targets ?context t =
   let env =
     match (context : Context.t option) with
-    | None -> Lazy.force Env.initial_env
+    | None -> Env.initial ()
     | Some c -> c.env
   in
   let targets = Path.Set.to_list targets in
diff --git a/src/context.ml b/src/context.ml
index b26d9562..42aeb868 100644
--- a/src/context.ml
+++ b/src/context.ml
@@ -33,7 +33,7 @@ type t =
   ; ocamlopt                : Path.t option
   ; ocamldep                : Path.t
   ; ocamlmklib              : Path.t
-  ; env                     : string array
+  ; env                     : Env.t
   ; env_extra               : string Env.Map.t
   ; findlib                 : Findlib.t
   ; findlib_toolchain       : string option
@@ -110,7 +110,8 @@ let opam_config_var ~env ~cache var =
     match Bin.opam with
     | None -> Fiber.return None
     | Some fn ->
-      Process.run_capture (Accept All) (Path.to_string fn) ~env
+      Process.run_capture (Accept All) (Path.to_string fn)
+        ~env:(Env.to_unix env)
         ["config"; "var"; var]
       >>| function
       | Ok s ->
@@ -124,7 +125,7 @@ let which ~cache ~path x =
 
 let create ~(kind : Kind.t) ~path ~base_env ~env_extra ~name ~merlin
       ~targets () =
-  let env = Env.extend_env ~env:base_env ~vars:env_extra in
+  let env = Env.extend base_env ~vars:env_extra in
   let opam_var_cache = Hashtbl.create 128 in
   (match kind with
    | Opam { root; _ } ->
@@ -145,7 +146,7 @@ let create ~(kind : Kind.t) ~path ~base_env ~env_extra ~name ~merlin
       match Sys.getenv "OCAMLFIND_CONF" with
       | s -> Fiber.return (Path.absolute s)
       | exception Not_found ->
-        Process.run_capture_line ~env Strict
+        Process.run_capture_line ~env:(Env.to_unix env) Strict
           (Path.to_string fn) ["printconf"; "conf"]
         >>| Path.absolute)
   in
@@ -207,7 +208,9 @@ let create ~(kind : Kind.t) ~path ~base_env ~env_extra ~name ~merlin
             | None -> args
             | Some s -> "-toolchain" :: s :: args
           in
-          Process.run_capture_lines ~env Strict (Path.to_string fn) args
+          Process.run_capture_lines
+            ~env:(Env.to_unix env)
+            Strict (Path.to_string fn) args
           >>| List.map ~f:Path.absolute
         | None ->
           (* If there no ocamlfind in the PATH, check if we have opam
@@ -230,7 +233,7 @@ let create ~(kind : Kind.t) ~path ~base_env ~env_extra ~name ~merlin
     Fiber.fork_and_join
       findlib_path
       (fun () ->
-         Process.run_capture_lines ~env Strict
+         Process.run_capture_lines ~env:(Env.to_unix env) Strict
            (Path.to_string ocamlc) ["-config"]
          >>| fun lines ->
          let open Result.O in
@@ -248,11 +251,11 @@ let create ~(kind : Kind.t) ~path ~base_env ~env_extra ~name ~merlin
       && version >= (4, 03, 0)
       && version <  (4, 05, 0) then
         let value =
-          match Env.get_env env "OCAMLPARAM" with
+          match Env.get_var env "OCAMLPARAM" with
           | None -> "color=always,_"
           | Some s -> "color=always," ^ s
         in
-        Env.extend_env ~env ~vars:((Env.Map.singleton "OCAMLPARAM" value)),
+        Env.extend env ~vars:(Env.Map.singleton "OCAMLPARAM" value),
         (Env.Map.add env_extra "OCAMLPARAM" value)
       else
         env,env_extra
@@ -270,7 +273,8 @@ let create ~(kind : Kind.t) ~path ~base_env ~env_extra ~name ~merlin
       ; for_host = host
       ; build_dir
       ; path
-      ; toplevel_path = Option.map (Env.get_env env "OCAML_TOPLEVEL_PATH") ~f:Path.absolute
+      ; toplevel_path =
+          Option.map (Env.get_var env "OCAML_TOPLEVEL_PATH") ~f:Path.absolute
 
       ; ocaml_bin  = dir
       ; ocaml      = (match which "ocaml" with Some p -> p | None -> prog_not_found_in_path "ocaml")
@@ -342,9 +346,9 @@ let create ~(kind : Kind.t) ~path ~base_env ~env_extra ~name ~merlin
 let opam_config_var t var = opam_config_var ~env:t.env ~cache:t.opam_var_cache var
 
 let default ?(merlin=true) ~targets () =
-  let env = Lazy.force Env.initial_env in
+  let env = Env.initial () in
   let path =
-    match Env.get_env env "PATH" with
+    match Env.get_var env "PATH" with
     | Some s -> Bin.parse_path s
     | None -> []
   in
@@ -387,7 +391,7 @@ let create_for_opam ?root ~targets ~switch ~name ?(merlin=false) () =
       | None -> Bin.path
       | Some s -> Bin.parse_path s
     in
-    let env = Lazy.force Env.initial_env in
+    let env = Env.initial () in
     create ~kind:(Opam { root; switch }) ~targets
       ~path ~base_env:env ~env_extra:vars ~name ~merlin ()
 
@@ -412,7 +416,7 @@ let install_ocaml_libdir t =
     (* If ocamlfind is present, it has precedence over everything else. *)
     match which t "ocamlfind" with
     | Some fn ->
-      (Process.run_capture_line ~env:t.env Strict
+      (Process.run_capture_line ~env:(Env.to_unix t.env) Strict
          (Path.to_string fn) ["printconf"; "destdir"]
        >>| fun s ->
        Some (Path.absolute s))
@@ -425,7 +429,7 @@ let env_for_exec t =
   let cwd = Sys.getcwd () in
   let extend_var var v =
     let v = Filename.concat cwd (Path.to_string v) in
-    match Env.get_env t.env var with
+    match Env.get_var t.env var with
     | None -> (var, v)
     | Some prev -> (var, sprintf "%s%c%s" v sep prev)
   in
@@ -444,7 +448,7 @@ let env_for_exec t =
         (Config.local_install_man_dir ~context:t.name)
     ]
   in
-  Env.extend_env ~env:t.env ~vars:(Env.Map.of_list_exn vars)
+  Env.to_unix (Env.extend t.env ~vars:(Env.Map.of_list_exn vars))
 
 let compiler t (mode : Mode.t) =
   match mode with
diff --git a/src/context.mli b/src/context.mli
index 921a6160..40027bf5 100644
--- a/src/context.mli
+++ b/src/context.mli
@@ -63,7 +63,7 @@ type t =
   ; ocamlmklib : Path.t
 
   ; (** Environment variables *)
-    env : string array
+    env : Env.t
 
   ; (** Diff between the base environment and [env] *)
     env_extra : string Env.Map.t
diff --git a/src/env.ml b/src/env.ml
index af18dbab..fbdccea0 100644
--- a/src/env.ml
+++ b/src/env.ml
@@ -1,9 +1,5 @@
 open Import
 
-let initial_env = lazy (
-  Lazy.force Colors.setup_env_for_colors;
-  Unix.environment ())
-
 module Var = struct
   type t = string
   let compare a b =
@@ -14,7 +10,39 @@ module Var = struct
 
 end
 
-let get_env env var =
+module Map = Map.Make(Var)
+
+type t =
+  { base : string array
+  ; extra : string Map.t
+  ; combined : string array Lazy.t
+  }
+
+let make ~base ~extra =
+  { base
+  ; extra
+  ; combined = lazy (
+      if Map.is_empty extra then
+        base
+      else
+        let imported =
+          Array.to_list base
+          |> List.filter ~f:(fun s ->
+            match String.index s '=' with
+            | None -> true
+            | Some i ->
+              let key = String.sub s ~pos:0 ~len:i in
+              not (Map.mem extra key))
+        in
+        List.rev_append
+          (List.map (Map.to_list extra)
+             ~f:(fun (k, v) -> sprintf "%s=%s" k v))
+          imported
+        |> Array.of_list
+    )
+  }
+
+let get_env_base env var =
   let rec loop i =
     if i = Array.length env then
       None
@@ -27,25 +55,28 @@ let get_env env var =
   in
   loop 0
 
-module Map = Map.Make(Var)
+let get_var t v =
+  match Map.find t.extra v with
+  | None -> get_env_base t.base v
+  | Some _ as v -> v
 
-let extend_env ~vars ~env =
-  if Map.is_empty vars then
-    env
-  else
-    let imported =
-      Array.to_list env
-      |> List.filter ~f:(fun s ->
-        match String.index s '=' with
-        | None -> true
-        | Some i ->
-          let key = String.sub s ~pos:0 ~len:i in
-          not (Map.mem vars key))
-    in
-    List.rev_append
-      (List.map (Map.to_list vars)
-         ~f:(fun (k, v) -> sprintf "%s=%s" k v))
-      imported
-    |> Array.of_list
+let to_unix t = Lazy.force t.combined
 
+let initial =
+  let i =
+    lazy (
+      make
+        ~base:(Lazy.force Colors.setup_env_for_colors;
+               Unix.environment ())
+        ~extra:Map.empty
+    ) in
+  fun () -> Lazy.force i
 
+let extend t ~vars =
+  make ~base:t.base
+    ~extra:(
+      Map.merge t.extra vars ~f:(fun _ v1 v2 ->
+        match v2 with
+        | Some _ -> v2
+        | None -> v1)
+    )
diff --git a/src/env.mli b/src/env.mli
index 4f8ca1dc..81b85e62 100644
--- a/src/env.mli
+++ b/src/env.mli
@@ -2,12 +2,17 @@ open Import
 
 module Var : sig
   type t = string
+  val compare : t -> t -> Ordering.t
 end
 
+type t
+
 module Map : Map.S with type key = Var.t
 
-val initial_env : string array Lazy.t
+val initial : unit -> t
 
-val extend_env : vars:string Map.t -> env:string array -> string array
+val to_unix : t -> string array
 
-val get_env : string array -> string -> string option
+val get_var : t -> Var.t -> string option
+
+val extend : t -> vars:string Map.t -> t
diff --git a/src/jbuild_load.ml b/src/jbuild_load.ml
index f7882a40..3564edcf 100644
--- a/src/jbuild_load.ml
+++ b/src/jbuild_load.ml
@@ -148,7 +148,8 @@ end
            in
          ]}
       *)
-      Process.run Strict ~dir:(Path.to_string dir) ~env:context.env
+      Process.run Strict ~dir:(Path.to_string dir)
+        ~env:(Env.to_unix context.env)
         (Path.to_string context.ocaml)
         args
       >>= fun () ->