In an apparent bid to pull traffic to its own Web site, Delta Air Lines announced a guarantee that its lowest fares will be on its own Web site.

The guarantee Delta announced Tuesday comes with a number of caveats. But it basically boils down to this: If a customer books a ticket at www.delta.com, then finds exactly the same Delta itinerary on another travel Web site by midnight of the same day that is at least $10 cheaper, he or she will be refunded the fare difference and get a $100 ticket voucher. Delta said the customer can instead get a ticket refund, no questions asked, if the same conditions are met within 24 hours.

"What they're trying to do is get more traffic to their Web site," said Tom Parsons, chief executive of a rival travel Web site, Bestfares.com. He said most big airlines have announced similar policies in recent years because they can save several dollars per ticket on fees that would otherwise be paid to outside ticket distributors and travel sites. "It's big bucks."