Round-trip fares are sometimes less expensive than one-way fares. If I book a round-trip and only use one part of the ticket, will there be a penalty?
While it may seem like an easy way to save a few bucks on airfare, flying only one part of a round-trip itinerary can get you into trouble with your airline.
The practice you're referring to is known as "throwaway ticketing." Most of the major airlines, including Alaska, American, Continental, Delta, Northwest, and US Airways, prohibit throwaway ticketing (or "the purchase and use of round-trip tickets for the purpose of one-way travel only," according to Continental's website) in their conditions of carriage. United is the only major carrier that does not explicitly prohibit throwaway ticketing.
Just because throwaway ticketing is prohibited doesn't necessarily mean you can't do it. With thousands of passengers each day, you may wonder if your airline will notice that you're not on your return flight. And it may not notice. However, if you are caught, you may be penalized. Possible penalties include deleting frequent flyer miles from your account and charging you for the difference between the fare paid and the fare for the itinerary you actually traveled. It's ultimately up to you to weigh the risks against the benefits.
A better way to save money on a one-way flight is to book with an airline that sells inexpensive one-way tickets. Low-fare carriers, including AirTran, ATA, Independence Air, JetBlue, Southwest, and Spirit, sell primarily one-way tickets, which means to fly a round-trip, customers must book two one-way flights. Of course, these carriers don't serve quite as many cities as the major legacy airlines, so this strategy will not work for every itinerary, but the carriers do serve most of the major U.S. cities as well as several airports in the Caribbean and Mexico.