diff --git a/var/spack/repos/builtin/packages/python/package.py b/var/spack/repos/builtin/packages/python/package.py
index 3e0f038768..8dc46f113d 100644
--- a/var/spack/repos/builtin/packages/python/package.py
+++ b/var/spack/repos/builtin/packages/python/package.py
@@ -34,6 +34,7 @@
 
 import spack
 from spack import *
+from spack.util.environment import is_system_path
 from spack.util.prefix import Prefix
 import spack.util.spack_json as sjson
 
@@ -543,7 +544,10 @@ def setup_dependent_environment(self, spack_env, run_env, dependent_spec):
         # where a system provided python is run against the standard libraries
         # of a Spack built python. See issue #7128
         spack_env.set('PYTHONHOME', self.home)
-        spack_env.prepend_path('PATH', os.path.dirname(self.command.path))
+
+        path = os.path.dirname(self.command.path)
+        if not is_system_path(path):
+            spack_env.prepend_path('PATH', path)
 
         python_paths = []
         for d in dependent_spec.traverse(
diff --git a/var/spack/repos/builtin/packages/tcl/package.py b/var/spack/repos/builtin/packages/tcl/package.py
index b1ddc5a141..15891af2e1 100644
--- a/var/spack/repos/builtin/packages/tcl/package.py
+++ b/var/spack/repos/builtin/packages/tcl/package.py
@@ -24,6 +24,7 @@
 ##############################################################################
 import os
 
+from spack.util.environment import is_system_path
 from spack import *
 
 
@@ -102,7 +103,9 @@ def setup_dependent_environment(self, spack_env, run_env, dependent_spec):
         # where a system provided tcl is run against the standard libraries
         # of a Spack built tcl. See issue #7128 that relates to python but
         # it boils down to the same situation we have here.
-        spack_env.prepend_path('PATH', os.path.dirname(self.command.path))
+        path = os.path.dirname(self.command.path)
+        if not is_system_path(path):
+            spack_env.prepend_path('PATH', path)
 
         tcl_paths = [join_path(self.prefix, self.tcl_builtin_lib_dir)]