Online airline services seem like they're made for a small-business traveler in a hurry. Log on, punch in your itinerary, and back comes a list of the cheapest fares available from every airline. It's one-stop shopping, right?

Well, not exactly, as our test of five popular Web travel sites found. You're probably better off if you stop at several of these one-stop shopping services. The services rarely quote the same price even when we punched in the exact same information during our tests, and the difference can mean hundreds of dollars.

What's more, the itinerary for the so-called best fares often varied considerably. From site to site, flights may leave hours from requested departure times. And the process offers little peace of mind. In the perpetual-motion world of modern airline prices, where prices change from minute to minute, you might miss a steal on one site while searching on another.

To get the lowdown on low fares, Frontier looked at unrestricted, round-trip offerings between some major U.S. cities on five popular Web services: Expedia.com, PreviewTravel.com, TheTrip.com, which provides travel data for Business Week online, ITN.com, and Travelocity.com. We looked at fares, routes, and how much trouble it was to use the site. Our comparison, conducted during the first week of September, assumed you were leaving Wednesday, Sept. 9 at around 6 a.m., and returning Thursday around 5 p.m. The results:

New York to Chicago
Best deal: The best bet was PreviewTravel's fare of $1,005 on American Airlines, leaving Newark Airport at 6:40 a.m. for O'Hare International, with a 4:30 p.m. return the next day.
Runner-up: Travelocity quoted $1,031 on American. This fare was found as late as the day of travel. It leaves LaGuardia Airport at 12 p.m. for O'Hare and returns to LaGuardia on United Air Lines at 5 p.m. Thursday.
Also-rans: ITN's $1,219 on Delta, TWA, and United was more expensive but at least offered a variety of suitable itineraries from JFK, LaGuardia, or Newark to O'Hare. The most expensive and least- convenient was TheTrip's $1,395 flight. You take TWA from JFK at 4:10 p.m. for O'Hare. You return on American leaving at 4:14 p.m. the next day, connect in Raleigh-Durham, and arrive in JFK at 9:40 p.m. Expedia came within a half an hour of our desired departure times on both United and American for travel between Newark and O'Hare, but pegged the fare at $1,449 -- almost 50% more than PreviewTravel quoted for one of the same carriers.

Philadelphia to Los Angeles
Best deal: Travelocity offered substantial savings at $1,402, found the day of travel. America West will take you from Philadelphia at 6:07 p.m. to Los Angeles International, arriving at 10 p.m. The next day, you leave a little early on United's 2 p.m. flight, which gets you back to Philadelphia at 10 p.m.
Runner-up: ITN's option is a little more expensive ($1,505) and a lot less convenient (change in Las Vegas to America West from US Airways with a one-hour layover. You leave at 10 a.m., and get in at 12:49 p.m.) It's the red-eye home, leaving at 9:30 p.m. to get in at 6:30 a.m.
Also-rans: PreviewTravel and TheTrip tied, offering US Airways' $1,836 fare. Expedia's best quote came in at a whopping $2,336 on United.

Dallas to Washington, D.C.
Best deal: Travelocity and Expedia ran neck and neck with fares of $1,368 and $1,370, respectively. Travelocity's itinerary has you leaving on United, flying into Dulles, and returning on Delta from National Airport. Expedia's alternative is on US Airways, into and out of National.
Runners-up: The other three sites all came up with $1,452 on American flying into Dulles. You have your pick of departure times.

Boston to Miami
Best deal: PreviewTravel led with a $1,082 fare on Continental Airlines. The return has a half hour stopover in Newark, which gets you back to Beantown by 9:30 p.m.
Runner-up: ITN came next with a $1,264 flight on American at 6:35 a.m., returning on US Airways at 7:25 p.m. with a one-hour stopover in Charlotte, N.C. You see Boston again around midnight.
Also-rans: Expedia offered a US Airways flight for $1,382. The Trip and Travelocity tied for a direct flight on American for $1,446.

New Orleans to Atlanta
Best deal: PreviewTravel, TheTrip, and ITN all offered direct flights on Airtran Airways for $326 with a range of departure times.
Runner-up: Travelocity gave a day-of-travel price on the same airline for $726. As noted below, we probably would have done better if we had asked sooner.
Also-ran: Expedia's best fare was $876 on Continental.

Why the big differences? One reason is that buying an airline ticket is like buying stocks these days -- the prices are changing every minute. In fact, says Antoine Toffa, president of TheTrip, airlines file more than 100,000 changes a day on domestic routes.

That may also explain why Expedia's selections were among the costliest for three out of the five trips. Suzan Levine, product manager for the site, insisted it was a random occurrence. "There is no reason why our prices are higher. Just as many time we'll have the lowest fares," she says, adding that a later search might produce a different set of choices. That's a fair point. We surveyed the sites consecutively -- the way a single business owner might do it -- rather than simultaneously.

So we went back to the computer yesterday and tried again, specifying some of the same flights for Tuesday, Sept. 22, and returning on Wednesday. We couldn't even get into Expedia this time, which the site attributed to heavy use. As for the remaining four sites, prices still weren't uniform, and in some cases, ITN and TheTrip were offering cheaper fares on the same flights cited by rivals at higher prices.

One possible reason is that the services don't all tap into the same database. Expedia gets its information from WorldSpan; Travelocity taps Sabre; while TheTrip, ITN, and PreviewTravel all use Apollo, albeit with separate software interfaces. So it's hardly a surprise that TheTrip and ITN often gave the same fares, carriers, and routes.

Less clear is the role of the airlines, which own all or part of these sites. The parent of American Airlines has a big stake in Travelocity. Northwest and Continental own WorldSpan, and United operates Apollo indirectly. The sites aren't subject to laws that prohibit travel agents and computerized reservation systems from rearranging the information to make one airline's options appear to match your travel criteria better than another. To their credit, though, there was no clear pattern of discrimination. For instance, Travelocity gave United the nod instead of American even on a flight between Washington, D.C., and Dallas/Ft. Worth, even though Dallas is American's hotly contested home turf.

So how should you play this game? Make sure you query at least two sites using different databases. As a backup, be sure to sign up for free E-mail offers that tell you about cheaper rates to your destination that pop up later. You'll suffer some junk mail, but you won't have that gnawing feeling you're missing a better deal.

Some general gripes: Each site makes you fill out a time-wasting profile form for those free E-mails, but presumably for marketing purposes as well. There are also some vexing individual quirks. For example, Travelocity's site is a headache if you change destinations. You have to go back to the home page and start again instead of just typing in a new city. PreviewTravel needs to condense its tedious series of screens to one. ITN and Expedia's sites, by contrast, are a breeze to use and give quick answers. That's more than you can say for some travel agents.

By Jeremy Quittner in New York
jeremy_quittner@businessweek.com