Got this from about.com:
Splenda Splenda is the brand name for a sucralose sweetener. Sucralose is made from natural sugar, by a process of chlorination. The addition of chlorine to the sugar molecule makes it unusable by the body, and it is not broken down by your metabolism. Splenda (sucralose) is 600 times sweeter than natural sugar and contains no calories.
Using Splenda, you might as well be eating bleach ('cause that's one of Splenda's ingredients). It's also used during processing.