urldecoding

We are receiving from flash a request with the colour Weiß in the query string, so the query string looks something like ?colour=Wei%DF
%DF is the hex equivalent of 223 which is the ascii code for the German character.
When you view the raw url you see what was sent but when you try request.QueryString.ToString() You see
"colour=Wei%ufffd"
and I have not found a way to get back to Weiß
We are going to have to write our own query parser to process the raw url and use
Response.Write(HttpUtility.UrlDecode("Wei%DF", Encoding.Default))

I would like to know what is going on.

Thank ...