diff --git a/docs/change-log.md b/docs/change-log.md
index cb2fabf0..de0b3769 100644
--- a/docs/change-log.md
+++ b/docs/change-log.md
@@ -24,6 +24,7 @@
 #### Bug fixes
 
 - API: `m.route.set()` causes all mount points to be redrawn ([#1592](https://github.com/MithrilJS/mithril.js/pull/1592))
+- API: If a user sets the Content-Type header within a request's options, that value will be the entire header value rather than being appended to the default value ([#1924](https://github.com/MithrilJS/mithril.js/pull/1924))
 
 ---
 
diff --git a/request/request.js b/request/request.js
index ac5f3391..7e4ec744 100644
--- a/request/request.js
+++ b/request/request.js
@@ -67,10 +67,10 @@ module.exports = function($window, Promise) {
 
 			xhr.open(args.method, args.url, typeof args.async === "boolean" ? args.async : true, typeof args.user === "string" ? args.user : undefined, typeof args.password === "string" ? args.password : undefined)
 
-			if (args.serialize === JSON.stringify && useBody) {
+			if (args.serialize === JSON.stringify && useBody && !(args.headers && args.headers.hasOwnProperty("Content-Type"))) {
 				xhr.setRequestHeader("Content-Type", "application/json; charset=utf-8")
 			}
-			if (args.deserialize === deserialize) {
+			if (args.deserialize === deserialize && !(args.headers && args.headers.hasOwnProperty("Accept"))) {
 				xhr.setRequestHeader("Accept", "application/json, text/*")
 			}
 			if (args.withCredentials) xhr.withCredentials = args.withCredentials
diff --git a/test-utils/tests/test-xhrMock.js b/test-utils/tests/test-xhrMock.js
index 8bdfcf21..8c64a308 100644
--- a/test-utils/tests/test-xhrMock.js
+++ b/test-utils/tests/test-xhrMock.js
@@ -72,6 +72,14 @@ o.spec("xhrMock", function() {
 			}
 			xhr.send("a=b")
 		})
+		o("Setting a header twice merges the header", function() {
+			// Source: https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/setRequestHeader
+			var xhr = new $window.XMLHttpRequest()
+			xhr.open("POST", "/test")
+			xhr.setRequestHeader("Content-Type", "foo")
+			xhr.setRequestHeader("Content-Type", "bar")
+			o(xhr.getRequestHeader("Content-Type")).equals("foo, bar")
+		})
 	})
 	o.spec("jsonp", function() {
 		o("works", function(done) {
diff --git a/test-utils/xhrMock.js b/test-utils/xhrMock.js
index d3bbf5b8..379bce28 100644
--- a/test-utils/xhrMock.js
+++ b/test-utils/xhrMock.js
@@ -17,7 +17,17 @@ module.exports = function() {
 			var headers = {}
 			var aborted = false
 			this.setRequestHeader = function(header, value) {
-				headers[header] = value
+				/*
+				 the behavior of setHeader is not your expected setX API.
+				 If the header is already set, it'll merge with whatever you add
+				 rather than overwrite
+				 Source: https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/setRequestHeader
+				 */
+				if (headers[header]) {
+					headers[header] += ", " + value;
+				} else {
+					headers[header] = value
+				}
 			}
 			this.getRequestHeader = function(header) {
 				return headers[header]