this blog helped me to find the solution:
a snippet from my code:
using System.IO.Compression;
…
SqlBytes bytes = sqlDataReader.GetSqlBytes(sqlDataReader.GetOrdinal(“xmlFields”));
Stream stream = bytes.Stream;
for (int i = 0; i < 14; i++)
stream.ReadByte();
DeflateStream deflateStream = new DeflateStream(stream, CompressionMode.Decompress, true);
StreamReader reader = new StreamReader(deflateStream);
string fields = reader.ReadToEnd();