diff --git a/lib/pkg-config.rb b/lib/pkg-config.rb
index 542d67c..af5a696 100644
--- a/lib/pkg-config.rb
+++ b/lib/pkg-config.rb
@@ -264,6 +264,14 @@ def compute_native_pkg_config_prefix
     end
 
     def compute_default_path
+      libdir = ENV["PKG_CONFIG_LIBDIR"]
+      if libdir
+        return [
+          with_config("pkg-config-path") || ENV["PKG_CONFIG_PATH"],
+          libdir,
+        ].compact.join(SEPARATOR)
+      end
+
       default_paths = nil
       if native_pkg_config
         pc_path = run_command(native_pkg_config.to_s,
@@ -315,9 +323,6 @@ def compute_default_path
         mingw_pkgconfig_path = Pathname.new(mingw_bin_path) + "../lib/pkgconfig"
         default_paths.unshift(mingw_pkgconfig_path.cleanpath.to_s)
       end
-      libdir = ENV["PKG_CONFIG_LIBDIR"]
-      default_paths.unshift(libdir) if libdir
-
       paths = []
       if /-darwin\d[\d\.]*\z/ =~ RUBY_PLATFORM and
         /\A(\d+\.\d+)/ =~ run_command("sw_vers", "-productVersion")
diff --git a/test/test-pkg-config.rb b/test/test-pkg-config.rb
index 67893b3..222e736 100644
--- a/test/test-pkg-config.rb
+++ b/test/test-pkg-config.rb
@@ -21,6 +21,21 @@ def setup
     @glib = PackageConfig.new("glib-2.0", options)
   end
 
+  def test_explicit_libdir_replaces_default_paths
+    original_libdir = ENV["PKG_CONFIG_LIBDIR"]
+    original_path = ENV["PKG_CONFIG_PATH"]
+    ENV["PKG_CONFIG_LIBDIR"] = "/isolated/pkgconfig"
+    ENV["PKG_CONFIG_PATH"] = "/extra/pkgconfig"
+    assert_equal([
+                   "/extra/pkgconfig",
+                   "/isolated/pkgconfig",
+                 ].join(File::PATH_SEPARATOR),
+                 PackageConfig.__send__(:compute_default_path))
+  ensure
+    ENV["PKG_CONFIG_LIBDIR"] = original_libdir
+    ENV["PKG_CONFIG_PATH"] = original_path
+  end
+
   def only_pkg_config_version(major, minor)
     pkg_config_version = `#{@pkgconf} --version`.chomp
     current_major, current_minor = pkg_config_version.split(".").collect(&:to_i)