I ended up with writing a C# helper class:

using System.Net;

using WatiN.Core;

using SHDocVw;

using Microsoft.VisualStudio.TestTools.UnitTesting;

using System.Globalization;

namespace Test

{

    public class NavigationObserver

    {

        private HttpStatusCode _statusCode;

        public NavigationObserver(IE ie)

        {

            InternetExplorer internetExplorer = (InternetExplorer)ie.InternetExplorer;

            internetExplorer.NavigateError += new DWebBrowserEvents2_NavigateErrorEventHandler(IeNavigateError);

        }

        public void ShouldHave(HttpStatusCode expectedStatusCode)

        {

            if (!_statusCode.Equals(expectedStatusCode))

            {

                Assert.Fail(string.Format(CultureInfo.InvariantCulture, "Wrong status code. Expected {0}, but was {1}",

                    expectedStatusCode, _statusCode));

            }

        }

        private void IeNavigateError(object pDisp, ref object URL, ref object Frame, ref object StatusCode, ref bool Cancel)

        {

            _statusCode = (HttpStatusCode)StatusCode;

        }

    }

}

using (IE ie = new IE())

{

    NavigationObserver observer = new NavigationObserver(ie);

    ie.GoTo("http://some.where.com");

    observer.ShouldHave(HttpStatusCode.NotFound);

}

public class Navigator
{
    private IE _ie;

    public Navigator(IE ie) { _ie = ie; }

    public void Post(Uri baseUri, params KeyValuePair<string, object>[] postData)     {         object flags = null;         object targetFrame = null;         object headers = "Content-Type: application/x-www-form-urlencoded" + Convert.ToChar(10) + Convert.ToChar(13);         object postDataBytes = MakeByteStreamOf(postData);         object resourceLocator = baseUri.ToString();         IWebBrowser2 browser = (IWebBrowser2)_ie.InternetExplorer;         browser.Navigate2(ref resourceLocator, ref flags, ref targetFrame, ref postDataBytes, ref headers);         _ie.WaitForComplete();     }

    private static byte[] MakeByteStreamOf(KeyValuePair<string, object>[] postData)     {         StringBuilder sb = new StringBuilder(); if (postData.Length > 0) {         foreach (KeyValuePair<string, object> postDataEntry in postData)         {             sb.Append(postDataEntry.Key).Append('=').Append(postDataEntry.Value).Append('&');         }         sb.Remove(sb.Length - 1, 1); }         return ASCIIEncoding.ASCII.GetBytes(sb.ToString());     } } For example, I can use it like so:

using (IE ie = new IE())

{

    Navigator navigator = new Navigator(ie);

    navigator.Post(new Uri("http://www.foo.com/"), new KeyValuePair<string, object>("p", 1));

    Assert.AreEqual("OK", ie.Text);

}