Using server.encode and server.decode do not create strings that MATCH !

I have a string:
cvqg/1ICe7U32syIFcyJPNaHYGsoLz7EqUE2+/0mwxU= If I encode that string with server.encode, I get:
cvqg%2f1ICe7U32syIFcyJPNaHYGsoLz7EqUE2%2b%2f0mwxU%3d When I turn around and server.decode that same string, I don't get the original value, instead I get:
cvqg/1ICe7U32syIFcyJPNaHYGsoLz7EqUE2 /0mwxU= The differences are subtle but important....  It is dropping the "+" character.  How can I get it to stop this or how can I get around this problem.
Note also that this works perfectly fine here:  http://meyerweb.com/eric/tools/dencoder/ so it is entirely a problem with ASP.NET.