diff --git a/src/core/opamFilename.ml b/src/core/opamFilename.ml
index d881a9bf530..b44ba762da7 100644
--- a/src/core/opamFilename.ml
+++ b/src/core/opamFilename.ml
@@ -376,7 +376,7 @@ let with_flock flag ?dontblock file f =
           else
             "/dev/null"
         in
-        let ch = Pervasives.open_out null in
+        let ch = Pervasives.open_out_bin null in
         Unix.descr_of_out_channel ch, Some ch
       | fd ->
         fd, None
diff --git a/src/core/opamSystem.ml b/src/core/opamSystem.ml
index 21391ee5bfc..ca50faba823 100644
--- a/src/core/opamSystem.ml
+++ b/src/core/opamSystem.ml
@@ -565,7 +565,7 @@ module Tar = struct
 
   let guess_type f =
     try
-      let ic = open_in f in
+      let ic = open_in_bin f in
       let c1 = input_char ic in
       let c2 = input_char ic in
       close_in ic;
@@ -611,7 +611,7 @@ end
 module Zip = struct
   let is_archive f =
     try
-      let ic = open_in f in
+      let ic = open_in_bin f in
       let c1 = input_char ic in
       let c2 = input_char ic in
       let c3 = input_char ic in
diff --git a/src/repository/opamVCS.ml b/src/repository/opamVCS.ml
index cc3941779b5..55f8a77afb8 100644
--- a/src/repository/opamVCS.ml
+++ b/src/repository/opamVCS.ml
@@ -109,7 +109,7 @@ module Make (VCS: VCS) = struct
       in
       let stdout_file =
         let f = OpamSystem.temp_file "rsync-files" in
-        let fd = open_out f in
+        let fd = open_out_bin f in
         List.iter (fun s -> output_string fd s; output_char fd '\n') files;
         close_out fd;
         f