diff --git a/Source/Siesta/Request/RequestCreation.swift b/Source/Siesta/Request/RequestCreation.swift
index cf90a124..ac3e656b 100644
--- a/Source/Siesta/Request/RequestCreation.swift
+++ b/Source/Siesta/Request/RequestCreation.swift
@@ -13,6 +13,15 @@ public extension Resource
 
     /**
       Convenience method to initiate a request with a body containing arbitrary data.
+      - Parameter method: The HTTP method of the request.
+      - Parameter data: The body of the request.
+      - Parameter contentType: The value for the request’s `Content-Type` header. The priority order is as follows:
+          - any content-type set in `Configuration.mutateRequests(...)` overrides
+          - any content-type set in `requestMutation`, which overrides
+          - this parameter, which overrides
+          - any content-type set with `Configuration.headers`.
+      - Parameter requestMutation: Allows you to override details fo the HTTP request before it is sent.
+          See `request(_:requestMutation:)`.
     */
     public func request(
             _ method:        RequestMethod,
@@ -25,9 +34,8 @@ public extension Resource
             {
             underlyingRequest in
 
-            underlyingRequest.addValue(contentType, forHTTPHeaderField: "Content-Type")
+            underlyingRequest.setValue(contentType, forHTTPHeaderField: "Content-Type")
             underlyingRequest.httpBody = data
-
             requestMutation(&underlyingRequest)
             }
         }
diff --git a/Tests/Functional/RequestSpec.swift b/Tests/Functional/RequestSpec.swift
index a986c780..4a9f2d92 100644
--- a/Tests/Functional/RequestSpec.swift
+++ b/Tests/Functional/RequestSpec.swift
@@ -541,6 +541,37 @@ class RequestSpec: ResourceSpecBase
                         }
                     }
                 }
+
+            it("overrides any Content-Type set in configuration headers")
+                {
+                service().configure { $0.headers["Content-Type"] = "frotzle/ooglatz" }
+                _ = stubRequest(resource, "POST")
+                    .withHeader("Content-Type", "application/json")
+                awaitNewData(resource().request(.post, json: ["foo": "bar"]))
+                }
+
+            it("lets ad hoc request mutation override the Content-Type")
+                {
+                _ = stubRequest(resource, "POST")
+                    .withHeader("Content-Type", "person/json")
+                let req = resource().request(.post, json: ["foo": "bar"])
+                    { $0.setValue("person/json", forHTTPHeaderField: "Content-Type") }
+                awaitNewData(req)
+                }
+
+            it("lets configured mutators override the Content-Type")
+                {
+                service().configure
+                    {
+                    $0.mutateRequests
+                        { $0.setValue("argonaut/json", forHTTPHeaderField: "Content-Type") }  // This one wins, even though...
+                    }
+
+                _ = stubRequest(resource, "POST").withHeader("Content-Type", "argonaut/json")
+                let req = resource().request(.post, json: ["foo": "bar"])                     // ...request(json:) sets it to "application/json"...
+                    { $0.setValue("person/json", forHTTPHeaderField: "Content-Type") }        // ...and ad hoc mutation overrides that.
+                awaitNewData(req)
+                }
             }
 
         describe("chained()")