“The requirement is among the measures we take to ensure the validity and security of bookings made on our points-of-sale,” Ms. Deines said.

Some airlines do the same thing, and restrict purchases on their foreign Web sites to customers in those countries, said Keith Melnick, executive vice president of corporate development at Kayak.com and a former revenue manager at American Airlines. “In airline speak,” he wrote in an e-mail message, “the airlines differentiate their pricing based on the point-of-sale (P.O.S.). However, it is generally not a good idea to start searching the site in these other countries since the ones that do differentiate pricing by P.O.S. don’t want you to do this and will prevent you from purchasing by requiring a local credit card.”

When this is the case, there are few work-arounds. If you are visiting friends in a foreign country, see if they can make a reservation for you. Likewise if you’re traveling on business abroad, perhaps your company’s foreign office can arrange your travel. You also might try contacting a travel agent in the area you’re planning to visit and ask them to make the purchase for you. The savings may outweigh any fees charged by the agent for processing the transaction.

While country-domain hopping may uncover deals for rental cars and domestic flights in those countries, it doesn’t seem to work as well for hotel chains. “The large chains like Marriott have implemented a single image inventory to ensure rate parity across all channels,” said Ram Badrinathan, a Mumbai-based senior analyst at PhoCusWright, a travel consulting and research company.

Bargain hunters should also watch out for hidden fees. Car rental agencies may charge foreign customers more for liability coverage. Web sites often assume that you are a resident of that country when you book online, so the additional cost may not appear until you pick up the car.

Also, the foreign spinoffs of airline travel sites may require that the trip originate from that country, making round-trip searches of little use to American travelers. Still, those sites may be useful if you’re planning to travel within that country. A recent search on Kayak.com for the lowest airfare between Paris and Nice found a $171 fare on Air France. The same search on Kayak.fr turned up a 100-euro ticket on easyJet.

Those sites might also come in handy for finding a cheap side trip. A recent glance at Travelocity.co.uk highlighted four-star hotel offers in Rome starting at 48 euros a night and vacation packages in Krakow, Poland. Meanwhile, Travelocity.com was advertising deals in Florida, New York and Mexico.

So while searching the foreign version of a travel Web site might not always lead to the lowest price, it might just lead you to a destination you never would have thought to visit.