diff --git a/eio_posix.opam b/eio_posix.opam
index 9d843811..8aa4d398 100644
--- a/eio_posix.opam
+++ b/eio_posix.opam
@@ -32,3 +32,4 @@ build: [
   ]
 ]
 dev-repo: "git+https://github.com/ocaml-multicore/eio.git"
+available: [os != "win32"]
diff --git a/eio_posix.opam.template b/eio_posix.opam.template
new file mode 100644
index 00000000..3da821cb
--- /dev/null
+++ b/eio_posix.opam.template
@@ -0,0 +1 @@
+available: [os != "win32"]