diff --git a/lib/core/mergeConfig.js b/lib/core/mergeConfig.js
index bb5e8d5..8e1ce61 100644
--- a/lib/core/mergeConfig.js
+++ b/lib/core/mergeConfig.js
@@ -15,13 +15,23 @@ module.exports = function mergeConfig(config1, config2) {
   config2 = config2 || {};
   var config = {};
 
-  utils.forEach(['url', 'method', 'params', 'data'], function valueFromConfig2(prop) {
+  var valueFromConfig2Keys = ['url', 'method', 'params', 'data'];
+  var mergeDeepPropertiesKeys = ['headers', 'auth', 'proxy'];
+  var defaultToConfig2Keys = [
+    'baseURL', 'url', 'transformRequest', 'transformResponse', 'paramsSerializer',
+    'timeout', 'withCredentials', 'adapter', 'responseType', 'xsrfCookieName',
+    'xsrfHeaderName', 'onUploadProgress', 'onDownloadProgress',
+    'maxContentLength', 'validateStatus', 'maxRedirects', 'httpAgent',
+    'httpsAgent', 'cancelToken', 'socketPath'
+  ];
+
+  utils.forEach(valueFromConfig2Keys, function valueFromConfig2(prop) {
     if (typeof config2[prop] !== 'undefined') {
       config[prop] = config2[prop];
     }
   });
 
-  utils.forEach(['headers', 'auth', 'proxy'], function mergeDeepProperties(prop) {
+  utils.forEach(mergeDeepPropertiesKeys, function mergeDeepProperties(prop) {
     if (utils.isObject(config2[prop])) {
       config[prop] = utils.deepMerge(config1[prop], config2[prop]);
     } else if (typeof config2[prop] !== 'undefined') {
@@ -33,13 +43,25 @@ module.exports = function mergeConfig(config1, config2) {
     }
   });
 
-  utils.forEach([
-    'baseURL', 'url', 'transformRequest', 'transformResponse', 'paramsSerializer',
-    'timeout', 'withCredentials', 'adapter', 'responseType', 'xsrfCookieName',
-    'xsrfHeaderName', 'onUploadProgress', 'onDownloadProgress', 'maxContentLength',
-    'validateStatus', 'maxRedirects', 'httpAgent', 'httpsAgent', 'cancelToken',
-    'socketPath'
-  ], function defaultToConfig2(prop) {
+  utils.forEach(defaultToConfig2Keys, function defaultToConfig2(prop) {
+    if (typeof config2[prop] !== 'undefined') {
+      config[prop] = config2[prop];
+    } else if (typeof config1[prop] !== 'undefined') {
+      config[prop] = config1[prop];
+    }
+  });
+
+  var axiosKeys = valueFromConfig2Keys
+    .concat(mergeDeepPropertiesKeys)
+    .concat(defaultToConfig2Keys);
+
+  var otherKeys = Object
+    .keys(config2)
+    .filter(function filterAxiosKeys(key) {
+      return axiosKeys.indexOf(key) === -1;
+    });
+
+  utils.forEach(otherKeys, function otherKeysDefaultToConfig2(prop) {
     if (typeof config2[prop] !== 'undefined') {
       config[prop] = config2[prop];
     } else if (typeof config1[prop] !== 'undefined') {
diff --git a/test/specs/core/mergeConfig.spec.js b/test/specs/core/mergeConfig.spec.js
index c6564ae..2e043d5 100644
--- a/test/specs/core/mergeConfig.spec.js
+++ b/test/specs/core/mergeConfig.spec.js
@@ -64,4 +64,9 @@ describe('core::mergeConfig', function() {
     var merged = mergeConfig(defaults, { timeout: 123 });
     expect(merged.timeout).toEqual(123);
   });
+
+  it('should allow setting custom options', function() {
+    var merged = mergeConfig(defaults, { foo: 'bar' });
+    expect(merged.foo).toEqual('bar');
+  });
 });