diff --git a/lib_eio/unix/eio_unix.ml b/lib_eio/unix/eio_unix.ml
index 14f81819..dadcf14d 100644
--- a/lib_eio/unix/eio_unix.ml
+++ b/lib_eio/unix/eio_unix.ml
@@ -47,4 +47,25 @@ module Stdenv = struct
     debug : Eio.Debug.t;
     backend_id: string;
   >
+
+  let with_env
+    ?stdin ?stdout ?stderr ?net ?domain_mgr
+    ?process_mgr ?clock ?mono_clock ?fs ?cwd
+    ?secure_random ?debug ?backend_id (env : base) : base
+  =
+    object
+      method stdin = Option.value ~default:env#stdin stdin
+      method stdout = Option.value ~default:env#stdout stdout 
+      method stderr = Option.value ~default:env#stderr stderr 
+      method net = Option.value ~default:env#net net 
+      method domain_mgr = Option.value ~default:env#domain_mgr domain_mgr 
+      method process_mgr = Option.value ~default:env#process_mgr process_mgr
+      method clock = Option.value ~default:env#clock clock
+      method mono_clock = Option.value ~default:env#mono_clock mono_clock
+      method fs = Option.value ~default:env#fs fs
+      method cwd = Option.value ~default:env#cwd cwd
+      method secure_random = Option.value ~default:env#secure_random secure_random 
+      method debug = Option.value ~default:env#debug debug
+      method backend_id = Option.value ~default:env#backend_id backend_id
+    end
 end
diff --git a/lib_eio/unix/eio_unix.mli b/lib_eio/unix/eio_unix.mli
index 05dfcb99..80ab1c75 100644
--- a/lib_eio/unix/eio_unix.mli
+++ b/lib_eio/unix/eio_unix.mli
@@ -89,6 +89,26 @@ module Stdenv : sig
   (** The common set of features provided by all traditional operating systems (BSDs, Linux, Mac, Windows).
 
       You can use the functions in {!Eio.Stdenv} to access these fields if you prefer. *)
+
+  val with_env :
+    ?stdin:source_ty r ->
+    ?stdout:sink_ty r ->
+    ?stderr:sink_ty r ->
+    ?net:[ `Generic | `Unix ] Eio.Net.ty r ->
+    ?domain_mgr:Eio.Domain_manager.ty r ->
+    ?process_mgr:Process.mgr_ty r ->
+    ?clock:float Eio.Time.clock_ty r ->
+    ?mono_clock:Eio.Time.Mono.ty r ->
+    ?fs:Eio.Fs.dir_ty Eio.Path.t ->
+    ?cwd:Eio.Fs.dir_ty Eio.Path.t ->
+    ?secure_random:Eio.Flow.source_ty r ->
+    ?debug:Eio.Debug.t ->
+    ?backend_id:string ->
+    base ->
+    base
+  (** [with_env env] allows you to replace parts of [env] with new resources.
+      
+      By default it will select the resource from [env]. *)
 end
 
 (** API for Eio backends only. *)