diff --git a/cibuildwheel/macos.py b/cibuildwheel/macos.py
index 25bf78a7..fa07ca12 100644
--- a/cibuildwheel/macos.py
+++ b/cibuildwheel/macos.py
@@ -63,6 +63,7 @@ def build(project_dir, output_dir, test_command, test_requires, test_extras, bef
                 call(['sudo', 'tar', '-C', '/Library/Frameworks/Python.framework/Versions/%s/' % config.version, '-xmf', '/tmp/python-patch.tar.gz'])
 
         installation_bin_path = '/Library/Frameworks/Python.framework/Versions/{}/bin'.format(config.version)
+        assert os.path.exists(os.path.join(installation_bin_path, 'python3' if config.version[0] == '3' else 'python'))
 
         # Python bin folders on Mac don't symlink python3 to python, so we do that
         # so `python` and `pip` always point to the active configuration.
@@ -89,6 +90,7 @@ def build(project_dir, output_dir, test_command, test_requires, test_extras, bef
 
         # install pip & wheel
         call(['python', get_pip_script, '--no-setuptools', '--no-wheel'], env=env)
+        assert os.path.exists(os.path.join(installation_bin_path, 'pip'))
         call(['pip', '--version'], env=env)
         call(['pip', 'install', '--upgrade', 'setuptools'], env=env)
         call(['pip', 'install', 'wheel'], env=env)
diff --git a/cibuildwheel/windows.py b/cibuildwheel/windows.py
index a9fc7d92..6563071d 100644
--- a/cibuildwheel/windows.py
+++ b/cibuildwheel/windows.py
@@ -100,22 +100,13 @@ def build(project_dir, output_dir, test_command, test_requires, test_extras, bef
 
     python_configurations = get_python_configurations(build_selector)
     for config in python_configurations:
+        # install Python
         config_python_path = get_python_path(config)
         simple_shell([nuget, "install"] + get_nuget_args(config))
-        if not os.path.exists(os.path.join(config_python_path, 'Scripts', 'pip.exe')):
-            simple_shell([os.path.join(config_python_path, 'python.exe'), get_pip_script ])
-
-        # check python & pip exist for this configuration
         assert os.path.exists(os.path.join(config_python_path, 'python.exe'))
-        assert os.path.exists(os.path.join(config_python_path, 'Scripts', 'pip.exe'))
-
-        # setup dirs
-        if os.path.exists(built_wheel_dir):
-            shutil.rmtree(built_wheel_dir)
-        os.makedirs(built_wheel_dir)
 
+        # set up PATH and environment variables for run_with_env
         env = os.environ.copy()
-        # set up environment variables for run_with_env
         env['PYTHON_VERSION'] = config.version
         env['PYTHON_ARCH'] = config.arch
         env['PATH'] = os.pathsep.join([
@@ -123,17 +114,30 @@ def build(project_dir, output_dir, test_command, test_requires, test_extras, bef
             os.path.join(config_python_path, 'Scripts'),
             env['PATH']
         ])
+        # update env with results from CIBW_ENVIRONMENT
         env = environment.as_dictionary(prev_environment=env)
 
         # for the logs - check we're running the right version of python
+        shell(['where', 'python'], env=env)
         shell(['python', '--version'], env=env)
         shell(['python', '-c', '"import struct; print(struct.calcsize(\'P\') * 8)\"'], env=env)
 
+        # make sure pip is installed
+        if not os.path.exists(os.path.join(config_python_path, 'Scripts', 'pip.exe')):
+            simple_shell([os.path.join(config_python_path, 'python.exe'), get_pip_script], env=env)
+        assert os.path.exists(os.path.join(config_python_path, 'Scripts', 'pip.exe'))
+
         # prepare the Python environment
         shell(['python', '-m', 'pip', 'install', '--upgrade', 'pip'], env=env)
+        shell(['pip', '--version'], env=env)
         shell(['pip', 'install', '--upgrade', 'setuptools'], env=env)
         shell(['pip', 'install', 'wheel'], env=env)
 
+        # setup dirs
+        if os.path.exists(built_wheel_dir):
+            shutil.rmtree(built_wheel_dir)
+        os.makedirs(built_wheel_dir)
+
         # run the before_build command
         if before_build:
             before_build_prepared = prepare_command(before_build, project=abs_project_dir)