Thanks mate. I wasn't aware of that one so I tried. However, there are still some problems, so I'll just post some code in here in hope that someone might shed some light on what I'm doing wrong.

A bit info first: the code is from a function supposed to serialize an object, transform the XML using XSLT and finally send it as http post. The code should also receive an XML response from the remote service, transform it with XSLT and finally deserialize it back into the object.

So here's the code, I'll comment on it as I go. Note that some of the code is there for testing purposes and is basically obsolete for the final version. Note also that I'm just a few weeks into C#, having coded VB since 98, prior to which I was sworn VFP man, thus ending up in this forum...

System.Net.WebRequest req = System.Net.WebRequest.Create(RemoteServiceURL);
req.Method = "POST";
req.ContentType = "text/xml";
// Get the request stream to write to
System.IO.Stream streamWriter = (System.IO.Stream) req.GetRequestStream();

System.Xml.Serialization.XmlSerializer xmltrans = new System.Xml.Serialization.XmlSerializer(typeof(PaymentContainer));

// Create memory stream for Payment object serialization
MemoryStream memStream = new MemoryStream();

// Serialize using the memStream
xmltrans.Serialize(memStream ,Payment);
memStream.Seek(0, SeekOrigin.Begin);

//Create XPath document for XML transformation
XPathDocument myXPathDocument = new XPathDocument(memStream);
XslTransform myXslTransform = new XslTransform();

// Load XSLT for transformation
string xsltDoc = System.AppDomain.CurrentDomain.BaseDirectory.ToString();
xsltDoc = xsltDoc.Replace("/","\\");
xsltDoc += "PaymentSerialize.xsl";
myXslTransform.Load(xsltDoc);

// IDEALLY, AT THIS POINT THIS SHOULD DO THE TRICK
// http://msdn.microsoft.com/library/default.asp?url=/library/en-us/cpref/html/frlrfsystemxmlxslxsltransformclasstransformtopic16.asp
// Call transform method and pass on the stream acquired from GetRequestStream()
myXslTransform.Transform(myXPathDocument, null, streamWriter, null);

//but this doesn't work (no error, though) so I'll wait with closing the stream
//streamWriter.close();

//test the serialization of object by serializing and writing it to disk
string xsltTestDoc = System.AppDomain.CurrentDomain.BaseDirectory.ToString();
xsltTestDoc = xsltTestDoc.Replace("/","\\");
xsltTestDoc += "PaymentSerializeTestDoc.xml";
XmlTextWriter xwriter = new XmlTextWriter(xsltTestDoc , null);
myXslTransform.Transform(myXPathDocument, null, xwriter, null);
xwriter.Close();
// Data in test XML document looks fine


// Tried also with MemoryStream
MemoryStream memStreamToSend = new MemoryStream();
myXslTransform.Transform(myXPathDocument, null, memStreamToSend, null);
memStreamToSend.Seek(0, SeekOrigin.Begin);

// read the contents of the stream and write them manually to the http
byte[] byteArray = new byte[memStreamToSend.Length];
int count = memStreamToSend.Read(byteArray, 0, (int) memStreamToSend.Length);
UnicodeEncoding uniEncoding = new UnicodeEncoding();
char[] charArray = new char[uniEncoding.GetCharCount(byteArray, 0, count)];
streamWriter.Write(byteArray, 0, byteArray.Length);

// litte examining of the stream shows it has length=533 but all bytes are ''

streamWriter.Close();
System.Net.WebResponse response = req.GetResponse();

StreamReader streamR = new StreamReader(response.GetResponseStream(), Encoding.UTF8);
string xmlString = streamR.ReadToEnd().Trim();

// need to transform the XML response before deserializing

PaymentContainer PaymentReceipt = (PaymentContainer) xmltrans.Deserialize(response.GetResponseStream());