Hi
Could someone tell me the invalid characters that CAN NOT be used in a url?

I wish to use these characters for other reasons.

cheers

They can all be used.
But some need to be translated in another form first.

look here for a list of those codes:
http://www.w3schools.com/tags/ref_urlencode.asp

for example, a space would be replaced with %20.
Like
http://www.host.com/my example.html
=>
http://www.host.com/my%20example.html

it depends where in the URL and why

filenames or pseudo filenames (URL rewriting) cannot contain any of these " \ / : * ? " < > | "

and some characters have special meanings to servers/browsers so should be avoided outside of filenames " \ / ? & # : "