diff --git a/packages/-ember-data/tests/unit/adapters/json-api-adapter/ajax-options-test.js b/packages/-ember-data/tests/unit/adapters/json-api-adapter/ajax-options-test.js
index febd895919b..8b010f2ec67 100644
--- a/packages/-ember-data/tests/unit/adapters/json-api-adapter/ajax-options-test.js
+++ b/packages/-ember-data/tests/unit/adapters/json-api-adapter/ajax-options-test.js
@@ -7,7 +7,7 @@ import DS from 'ember-data';
 
 let Person, Place, store, adapter, env;
 
-module('unit/adapters/json-api-adapter/ajax-options - building requests', function(hooks) {
+module('unit/adapters/json-api-adapter/ajax-options - building requests with fetch', function(hooks) {
   hooks.beforeEach(function() {
     Person = { modelName: 'person' };
     Place = { modelName: 'place' };
@@ -89,4 +89,58 @@ module('unit/adapters/json-api-adapter/ajax-options - building requests', functi
       'headers assigned, Accept header not overwritten'
     );
   });
+
+  test('ajaxOptions() headers are set POST', function(assert) {
+    adapter.headers = {};
+    let url = 'example.com';
+    let type = 'POST';
+    let ajaxOptions = adapter.ajaxOptions(url, type, { data: { type: 'post' } });
+    let receivedHeaders = ajaxOptions.headers;
+
+    assert.deepEqual(
+      receivedHeaders,
+      {
+        Accept: 'application/vnd.api+json',
+        'content-type': 'application/vnd.api+json',
+      },
+      'headers assigned on POST'
+    );
+  });
+
+  test('ajaxOptions() does not override with existing headers["Content-Type"] POST', function(assert) {
+    adapter.headers = { 'Content-Type': 'application/x-www-form-urlencoded' };
+    let url = 'example.com';
+    let type = 'POST';
+    let ajaxOptions = adapter.ajaxOptions(url, type, { data: { type: 'post' } });
+    let receivedHeaders = ajaxOptions.headers;
+
+    assert.deepEqual(
+      receivedHeaders,
+      {
+        'Content-Type': 'application/x-www-form-urlencoded',
+        Accept: 'application/vnd.api+json',
+      },
+      'content-type header not overwritten'
+    );
+  });
+
+  test('ajaxOptions() can override with options.contentType POST', function(assert) {
+    adapter.headers = {};
+    let url = 'example.com';
+    let type = 'POST';
+    let ajaxOptions = adapter.ajaxOptions(url, type, {
+      contentType: 'application/x-www-form-urlencoded',
+      data: { type: 'post' },
+    });
+    let receivedHeaders = ajaxOptions.headers;
+
+    assert.deepEqual(
+      receivedHeaders,
+      {
+        'content-type': 'application/x-www-form-urlencoded',
+        Accept: 'application/vnd.api+json',
+      },
+      'content-type header overwritten'
+    );
+  });
 });
diff --git a/packages/-ember-data/tests/unit/adapters/rest-adapter/ajax-options-test.js b/packages/-ember-data/tests/unit/adapters/rest-adapter/ajax-options-test.js
index 9ebd157c783..86fae73662d 100644
--- a/packages/-ember-data/tests/unit/adapters/rest-adapter/ajax-options-test.js
+++ b/packages/-ember-data/tests/unit/adapters/rest-adapter/ajax-options-test.js
@@ -8,7 +8,7 @@ import DS from 'ember-data';
 
 var Person, Place, store, adapter, env;
 
-module('unit/adapters/rest-adapter/ajax-options - building requests', function(hooks) {
+module('unit/adapters/rest-adapter/ajax-options - building requests with fetch', function(hooks) {
   hooks.beforeEach(function() {
     Person = { modelName: 'person' };
     Place = { modelName: 'place' };
@@ -82,7 +82,6 @@ module('unit/adapters/rest-adapter/ajax-options - building requests', function(h
     let url = 'example.com';
     let type = 'GET';
     let ajaxOptions = adapter.ajaxOptions(url, type, { data: { key: 'value' } });
-    delete ajaxOptions.beforeSend;
 
     assert.deepEqual(ajaxOptions, {
       credentials: 'same-origin',
@@ -100,7 +99,6 @@ module('unit/adapters/rest-adapter/ajax-options - building requests', function(h
     let url = 'example.com';
     let type = 'POST';
     let ajaxOptions = adapter.ajaxOptions(url, type, { data: { key: 'value' } });
-    delete ajaxOptions.beforeSend;
 
     assert.deepEqual(ajaxOptions, {
       credentials: 'same-origin',
@@ -115,11 +113,55 @@ module('unit/adapters/rest-adapter/ajax-options - building requests', function(h
     });
   });
 
+  test('ajaxOptions() can provide own headers["Content-Type"]', function(assert) {
+    let url = 'example.com';
+    let type = 'POST';
+    let ajaxOptions = adapter.ajaxOptions(url, type, {
+      headers: {
+        'Content-Type': 'application/x-www-form-urlencoded',
+      },
+      data: { key: 'value' },
+    });
+
+    assert.deepEqual(ajaxOptions, {
+      credentials: 'same-origin',
+      data: { key: 'value' },
+      body: '{"key":"value"}',
+      type: 'POST',
+      method: 'POST',
+      headers: {
+        'Content-Type': 'application/x-www-form-urlencoded',
+      },
+      url: 'example.com',
+    });
+  });
+
+  test('ajaxOptions() can provide own contentType in options', function(assert) {
+    let url = 'example.com';
+    let type = 'POST';
+    let ajaxOptions = adapter.ajaxOptions(url, type, {
+      contentType: 'application/x-www-form-urlencoded',
+      data: { key: 'value' },
+    });
+
+    assert.deepEqual(ajaxOptions, {
+      contentType: 'application/x-www-form-urlencoded',
+      credentials: 'same-origin',
+      data: { key: 'value' },
+      body: '{"key":"value"}',
+      type: 'POST',
+      method: 'POST',
+      headers: {
+        'content-type': 'application/x-www-form-urlencoded',
+      },
+      url: 'example.com',
+    });
+  });
+
   test('ajaxOptions() empty data', function(assert) {
     let url = 'example.com';
     let type = 'POST';
     let ajaxOptions = adapter.ajaxOptions(url, type, {});
-    delete ajaxOptions.beforeSend;
 
     assert.deepEqual(ajaxOptions, {
       credentials: 'same-origin',
@@ -145,4 +187,47 @@ module('unit/adapters/rest-adapter/ajax-options - building requests', function(h
       return fetchPlacePromise;
     });
   });
+
+  module('ajax-options - ajax', function(hooks) {
+    hooks.beforeEach(function() {
+      adapter.set('useFetch', false);
+    });
+
+    hooks.afterEach(function() {
+      run(() => {
+        store.destroy();
+        env.container.destroy();
+      });
+    });
+
+    test('ajaxOptions() Content-Type is not set with ajax GET', function(assert) {
+      adapter.headers = {};
+
+      let url = 'example.com';
+      let type = 'GET';
+      let ajaxOptions = adapter.ajaxOptions(url, type, {});
+
+      assert.notOk(ajaxOptions.contentType, 'contentType not set with GET');
+    });
+
+    test('ajaxOptions() Content-Type is not set with ajax POST no data', function(assert) {
+      adapter.headers = {};
+
+      let url = 'example.com';
+      let type = 'POST';
+      let ajaxOptions = adapter.ajaxOptions(url, type, {});
+
+      assert.notOk(ajaxOptions.contentType, 'contentType not set with POST no data');
+    });
+
+    test('ajaxOptions() Content-Type is set with ajax POST with data', function(assert) {
+      adapter.headers = {};
+
+      let url = 'example.com';
+      let type = 'POST';
+      let ajaxOptions = adapter.ajaxOptions(url, type, { data: { type: 'post' } });
+
+      assert.equal(ajaxOptions.contentType, 'application/json; charset=utf-8', 'contentType is set with POST');
+    });
+  });
 });
diff --git a/packages/adapter/addon/json-api.js b/packages/adapter/addon/json-api.js
index e473453a413..d86da31278a 100644
--- a/packages/adapter/addon/json-api.js
+++ b/packages/adapter/addon/json-api.js
@@ -143,6 +143,8 @@ import { pluralize } from 'ember-inflector';
 const JSONAPIAdapter = RESTAdapter.extend({
   defaultSerializer: '-json-api',
 
+  _defaultContentType: 'application/vnd.api+json',
+
   /**
     @method ajaxOptions
     @private
@@ -152,9 +154,8 @@ const JSONAPIAdapter = RESTAdapter.extend({
     @return {Object}
   */
   ajaxOptions(url, type, options = {}) {
-    options.contentType = options.contentType || 'application/vnd.api+json';
-
     let hash = this._super(url, type, options);
+
     hash.headers['Accept'] = hash.headers['Accept'] || 'application/vnd.api+json';
 
     return hash;
diff --git a/packages/adapter/addon/rest.js b/packages/adapter/addon/rest.js
index 763f56a8234..16d64b6faa7 100644
--- a/packages/adapter/addon/rest.js
+++ b/packages/adapter/addon/rest.js
@@ -291,6 +291,8 @@ const hasNajax = typeof najax !== 'undefined';
 const RESTAdapter = Adapter.extend(BuildURLMixin, {
   defaultSerializer: '-rest',
 
+  _defaultContentType: 'application/json; charset=utf-8',
+
   fastboot: computed(function() {
     return getOwner(this).lookup('service:fastboot');
   }),
@@ -1093,14 +1095,21 @@ const RESTAdapter = Adapter.extend(BuildURLMixin, {
       options.headers = {};
     }
 
-    if (options.data && options.type !== 'GET') {
-      let contentType = options.contentType || 'application/json; charset=utf-8';
-      options.headers['content-type'] = contentType;
-    }
+    let contentType = options.contentType || this._defaultContentType;
 
     if (get(this, 'useFetch')) {
+      if (options.data && options.type !== 'GET') {
+        if (!options.headers['Content-Type'] && !options.headers['content-type']) {
+          options.headers['content-type'] = contentType;
+        }
+      }
       options = fetchOptions(options, this);
     } else {
+      // GET requests without a body should not have a content-type header
+      // and may be unexpected by a server
+      if (options.data && options.type !== 'GET') {
+        options = assign(options, { contentType });
+      }
       options = ajaxOptions(options, this);
     }
 
@@ -1370,7 +1379,6 @@ function ajaxOptions(options, adapter) {
 
   if (options.data && options.type !== 'GET') {
     options.data = JSON.stringify(options.data);
-    options.contentType = 'application/json; charset=utf-8';
   }
 
   options.beforeSend = function(xhr) {