diff --git a/README.rst b/README.rst
index 675c9a7fbf9f17f63f025ca843d8e985735b366b..689f5b672c26ab3a5411f19c53eb6abbf050b75b 100644
--- a/README.rst
+++ b/README.rst
@@ -7,6 +7,10 @@ Building
 --------
 
 The build system uses `Nix`_ which must be installed before anything can be built.
+Builds are tested against a particular nixpkgs revision.
+Start by exporting ``NIX_PATH`` to ensure you use the same revision::
+
+  $ export NIX_PATH=nixpkgs=https://github.com/NixOS/nixpkgs-channels/archive/$(cat nixpkgs.rev).tar.gz
 
 Documentation
 ~~~~~~~~~~~~~
diff --git a/nixpkgs.nix b/nixpkgs.nix
deleted file mode 100644
index ff369425e75e52f449c8899a31fc2b2feefc9ffa..0000000000000000000000000000000000000000
--- a/nixpkgs.nix
+++ /dev/null
@@ -1,9 +0,0 @@
-# Pin the deployment package-set to a specific version of nixpkgs.  This is
-# NixOS 19.09 as of Oct 2 2019.  There's nothing special about it.  It's just
-# recent at the time of development.  It can be upgraded when there is value
-# in doing so.  Meanwhile, our platform doesn't shift around beneath us in
-# surprising ways as time passes.
-import (builtins.fetchTarball {
-  url = "https://github.com/NixOS/nixpkgs-channels/archive/5d5cd70516001e79516d2ade8bcf31df208a4ef3.tar.gz";
-  sha256 = "042i081cfwdvcfp3q79219akypb53chf730wg0vwxlp21pzgns33";
-})
diff --git a/nixpkgs.rev b/nixpkgs.rev
new file mode 100644
index 0000000000000000000000000000000000000000..fff71dcaf5357d1897074f00f3350e328fd8b609
--- /dev/null
+++ b/nixpkgs.rev
@@ -0,0 +1 @@
+8bf142e001b6876b021c8ee90c2c7cec385fe8e9