You will get the mileage for the portion you fly. Mileage Addict is right that with the change fee, it is generally uneconomic to use the return at a later date. The airlines will tell you that if you do this they will somehow punish you. It's hard to enforce, but remember, thanks to Y2K, a lot of these folks have taken the opportunity to rewrite their software. They now know a great deal more about us than they did. If this were a recurring thing, they might pick up on it, and decide to do something. In addition to possibly trying to collect a one way fare (hard to do), they also have the right to limit participation in their frequent flyer program (a remedy they frequently remind us of in small print in their collateral material). Bottom line, you can do it and get your miles, but don't abuse it 'cause big brother is watching.