diff --git a/src/app/rpmostree-compose-builtin-tree.c b/src/app/rpmostree-compose-builtin-tree.c
index 5c319692..ac3bce3a 100644
--- a/src/app/rpmostree-compose-builtin-tree.c
+++ b/src/app/rpmostree-compose-builtin-tree.c
@@ -213,6 +213,7 @@ set_keyfile_string_array_from_json (GKeyFile    *keyfile,
 
 static gboolean
 install_packages_in_root (RpmOstreeTreeComposeContext  *self,
+                          RpmOstreeContext *ctx,
                           JsonObject      *treedata,
                           GFile           *yumroot,
                           char           **packages,
@@ -225,7 +226,6 @@ install_packages_in_root (RpmOstreeTreeComposeContext  *self,
   guint progress_sigid;
   GFile *contextdir = self->treefile_context_dirs->pdata[0];
   g_autoptr(RpmOstreeInstall) hifinstall = { 0, };
-  g_autoptr(RpmOstreeContext) ctx = NULL;
   HifContext *hifctx;
   gs_free char *ret_new_inputhash = NULL;
   g_autoptr(GKeyFile) treespec = g_key_file_new ();
@@ -245,9 +245,6 @@ install_packages_in_root (RpmOstreeTreeComposeContext  *self,
     }
 #endif
 
-  ctx = rpmostree_context_new_unprivileged (self->cachedir_dfd, cancellable, error);
-  if (!ctx)
-    goto out;
   hifctx = rpmostree_context_get_hif (ctx);
   if (opt_proxy)
     hif_context_set_http_proxy (hifctx, opt_proxy);
@@ -577,6 +574,8 @@ rpmostree_compose_builtin_tree (int             argc,
   gs_unref_object JsonParser *treefile_parser = NULL;
   gs_unref_variant_builder GVariantBuilder *metadata_builder = 
     g_variant_builder_new (G_VARIANT_TYPE ("a{sv}"));
+  g_autoptr(RpmOstreeContext) corectx = NULL;
+  g_autoptr(GHashTable) varsubsts = NULL;
   gboolean workdir_is_tmp = FALSE;
 
   self->treefile_context_dirs = g_ptr_array_new_with_free_func ((GDestroyNotify)g_object_unref);
@@ -676,6 +675,12 @@ rpmostree_compose_builtin_tree (int             argc,
       goto out;
     }
 
+  corectx = rpmostree_context_new_unprivileged (self->cachedir_dfd, cancellable, error);
+  if (!corectx)
+    goto out;
+
+  varsubsts = rpmostree_context_get_varsubsts (corectx);
+
   treefile_parser = json_parser_new ();
   if (!json_parser_load_from_file (treefile_parser,
                                    gs_file_get_path_cached (treefile_path),
@@ -708,9 +713,13 @@ rpmostree_compose_builtin_tree (int             argc,
       goto out;
     }
 
-  self->ref = g_strdup (_rpmostree_jsonutil_object_require_string_member (treefile, "ref", error));
-  if (!self->ref)
-    goto out;
+  { const char *input_ref = _rpmostree_jsonutil_object_require_string_member (treefile, "ref", error);
+    if (!input_ref)
+      goto out;
+    self->ref = _rpmostree_varsubst_string (input_ref, varsubsts, error);
+    if (!self->ref)
+      goto out;
+  }
 
   if (!ostree_repo_read_commit (repo, self->ref, &previous_root, &previous_checksum,
                                 cancellable, &temp_error))
@@ -809,7 +818,7 @@ rpmostree_compose_builtin_tree (int             argc,
 
   { gboolean unmodified = FALSE;
 
-    if (!install_packages_in_root (self, treefile, yumroot,
+    if (!install_packages_in_root (self, corectx, treefile, yumroot,
                                    (char**)packages->pdata,
                                    opt_force_nocache ? NULL : &unmodified,
                                    &new_inputhash,
diff --git a/src/libpriv/rpmostree-core.c b/src/libpriv/rpmostree-core.c
index 08c3530e..65ff2e74 100644
--- a/src/libpriv/rpmostree-core.c
+++ b/src/libpriv/rpmostree-core.c
@@ -404,6 +404,16 @@ rpmostree_treespec_to_variant (RpmOstreeTreespec *spec)
   return g_variant_ref (spec->spec);
 }
 
+GHashTable *
+rpmostree_context_get_varsubsts (RpmOstreeContext *context)
+{
+  GHashTable *r = g_hash_table_new_full (g_str_hash, g_str_equal, g_free, g_free);
+
+  g_hash_table_insert (r, g_strdup ("basearch"), g_strdup (hif_context_get_base_arch (context->hifctx)));
+
+  return r;
+}
+
 static gboolean
 enable_one_repo (RpmOstreeContext    *context,
                  GPtrArray           *sources,
diff --git a/src/libpriv/rpmostree-core.h b/src/libpriv/rpmostree-core.h
index 3afabaa6..54de1eb8 100644
--- a/src/libpriv/rpmostree-core.h
+++ b/src/libpriv/rpmostree-core.h
@@ -50,6 +50,8 @@ RpmOstreeTreespec *rpmostree_treespec_new_from_keyfile (GKeyFile *keyfile, GErro
 RpmOstreeTreespec *rpmostree_treespec_new_from_path (const char *path, GError  **error);
 RpmOstreeTreespec *rpmostree_treespec_new (GVariant   *variant);
 
+GHashTable *rpmostree_context_get_varsubsts (RpmOstreeContext *context);
+
 GVariant *rpmostree_treespec_to_variant (RpmOstreeTreespec *spec);
 const char *rpmostree_treespec_get_ref (RpmOstreeTreespec *spec);
 
diff --git a/src/libpriv/rpmostree-util.c b/src/libpriv/rpmostree-util.c
index eb12de44..457a67a7 100644
--- a/src/libpriv/rpmostree-util.c
+++ b/src/libpriv/rpmostree-util.c
@@ -121,6 +121,70 @@ rpmostree_mkdtemp (const char   *template,
   return ret;
 }
 
+/* Given a string of the form
+ * "bla blah ${foo} blah ${bar}"
+ * and a hash table of variables, substitute the variable values.
+ */
+char *
+_rpmostree_varsubst_string (const char *instr,
+                            GHashTable *substitutions,
+                            GError **error)
+{
+  const char *s;
+  const char *p;
+  /* Acts as a reusable buffer space */
+  g_autoptr(GString) varnamebuf = g_string_new ("");
+  g_autoptr(GString) result = g_string_new ("");
+
+  s = instr;
+  while ((p = strstr (s, "${")) != NULL)
+    {
+      const char *varstart = p + 2;
+      const char *varend = strchr (varstart, '}');
+      const char *value;
+      if (!varend)
+        {
+          g_set_error (error, G_IO_ERROR, G_IO_ERROR_FAILED,
+                       "Unclosed variable reference starting at %u bytes",
+                       (guint)(p - instr));
+          return NULL;
+        }
+
+      /* Append leading bytes */
+      g_string_append_len (result, s, p - s);
+
+      /* Get a NUL-terminated copy of the variable name */
+      g_string_truncate (varnamebuf, 0);
+      g_string_append_len (varnamebuf, varstart, varend - varstart);
+
+      value = g_hash_table_lookup (substitutions, varnamebuf->str);
+      if (!value)
+        {
+          g_set_error (error, G_IO_ERROR, G_IO_ERROR_FAILED,
+                       "Unknown variable reference ${%s}",
+                       varnamebuf->str);
+          return NULL;
+        }
+      /* Append the replaced value */
+      g_string_append (result, value);
+
+      /* On to the next */
+      s = varend+1;
+    }
+
+  if (s != instr)
+    {
+      char *r;
+      g_string_append_len (result, s, p - s);
+      /* Steal the C string, NULL out the GString since we freed it */
+      r = g_string_free (result, FALSE);
+      result = NULL;
+      return r;
+    }
+  else
+    return g_strdup (instr);
+}
+
 gboolean
 _rpmostree_util_enumerate_directory_allow_noent (GFile               *dirpath,
 						 const char          *queryargs,
diff --git a/src/libpriv/rpmostree-util.h b/src/libpriv/rpmostree-util.h
index 0adfa50a..76eb2217 100644
--- a/src/libpriv/rpmostree-util.h
+++ b/src/libpriv/rpmostree-util.h
@@ -47,6 +47,11 @@ gboolean rpmostree_mkdtemp (const char   *template,
                              int         *out_tmpdir_dfd,  /* allow-none */
                              GError     **error);
 
+char *
+_rpmostree_varsubst_string (const char *instr,
+                            GHashTable *substitutions,
+                            GError **error);
+
 gboolean
 _rpmostree_util_enumerate_directory_allow_noent (GFile               *dirpath,
 						 const char          *queryargs,
diff --git a/tests/compose/test-repo.json b/tests/compose/test-repo.json
index 83e7e90a..714e1d57 100644
--- a/tests/compose/test-repo.json
+++ b/tests/compose/test-repo.json
@@ -1,5 +1,5 @@
 {
-    "ref": "fedora/test",
+    "ref": "fedora/${basearch}/test",
 
     "repos": ["test-repo"],
 
diff --git a/tests/test-compose.sh b/tests/test-compose.sh
index 037ddcd4..0f3082ae 100755
--- a/tests/test-compose.sh
+++ b/tests/test-compose.sh
@@ -27,7 +27,12 @@ check_root_test
 # Remove once it doesn't happen anymore.
 unset G_DEBUG
 
-(arch | grep -q x86_64) || { echo 1>&2 "$0 can be run only on x86_64"; echo "1..0" ; exit 77; }
+arch=$(arch)
+if ! test "${arch}" = x86_64; then
+    echo 1>&2 "$0 can be run only on x86_64"; echo "1..0" ; exit 77
+fi
+
+testref=fedora/${arch}/test
 
 echo "1..4"
 
@@ -44,7 +49,7 @@ echo "ok dry run"
 
 rpm-ostree --repo=repo compose tree $SRCDIR/test-repo.json
 ostree --repo=repo refs >refs.txt
-assert_file_has_content refs.txt fedora/test
+assert_file_has_content refs.txt ${testref}
 
 echo "ok compose"