diff --git a/src/Composer/Console/Application.php b/src/Composer/Console/Application.php
index c0871bf4b..d65d2ca15 100644
--- a/src/Composer/Console/Application.php
+++ b/src/Composer/Console/Application.php
@@ -114,11 +114,11 @@ class Application extends BaseApplication
             $packageConfig['config'] = $composerConfig;
         }
 
-        $vendorDir = $packageConfig['config']['vendor-dir'];
+        $vendorDir = getenv('COMPOSER_VENDOR_DIR') ?: $packageConfig['config']['vendor-dir'];
         if (!isset($packageConfig['config']['bin-dir'])) {
             $packageConfig['config']['bin-dir'] = $vendorDir.'/bin';
         }
-        $binDir = $packageConfig['config']['bin-dir'];
+        $binDir = getenv('COMPOSER_BIN_DIR') ?: $packageConfig['config']['bin-dir'];
 
         // initialize repository manager
         $rm = new Repository\RepositoryManager();