Are some airlines more miserly than others when making discounted frequent-flier award seats available? It would appear so.

I recently checked available award seats on 24 routes for various summer and fall 2007 travel dates found that Delta Air Lines Inc. and US Airways Group Inc. often required more miles than other major carriers.

AMR Corp.'s American and UAL Corp.'s United were the most generous, with coach seats available at their lowest award levels on 14 of 24 trips checked. Continental Airlines Inc. and Northwest Airlines Corp. fell in the middle. US Airways, however, offered its lowest-priced award on only two of 24 itineraries; Delta on zero.

For a June 2-9 trip from Philadelphia to San Francisco and back, American, United and Continental all had 25,000-mile coach seats available, while US Airways offered nothing cheaper than 50,000 miles. Between Seattle and London for a later September trip, American and United both had their lowest-priced awards available, but Delta, Northwest and US Airways offered nothing less than 100,000-mile coach seats -- double their lowest price.

Travelers have long complained about the paucity of available frequent-flier-award seats and program changes that effectively raised the mileage cost of "free" seats. Airlines have added new, more-plentiful categories of "unrestricted" awards in recent years, but at double or more what had been the standard mileage price.

This spot check on availability, while far from a comprehensive inventory, does show what consumers are up against when trying to score seats. In recent years, more miles have been chasing fewer seats. A resurgence of fare-paying customers has left fewer seats empty, yet there are more miles in circulation because airlines have found selling miles to credit-card companies can be lucrative.

For summer travel to Europe, seats at the standard 50,000- or 60,000-mile levels were virtually impossible to find in our sampling, but airlines routinely had 100,000-mile unrestricted seats available for trips to London, Paris, Frankfurt and Brussels.

Ditto for Hawaii. A check of three routes to the islands for a June 2-9 trip found no seats at the standard 35,000-mile level, but plenty of seats at double the price.

Airlines are, by and large, free to run their frequent-flier programs as they see fit. The Department of Transportation suggests, but doesn't require, that carriers offer 5% to 10% of available seats in a market any time they advertise a sale price or frequent-flier award.

Most airlines do report the total percentage of travel bought with some sort of frequent-flier award, including upgrades and double-mileage tickets, and 2005 totals ran from 6% to 9%. Carriers won't say how much of that is for awards at the lowest price levels, however.

Craig Bruya of Seattle tried unsuccessfully last week to find two business-class seats on United to Paris for a late April trip at United's 80,000-mile "saver" price level. Curious, he looked for such seats all the way to the end of November and found none.

If "there isn't a single open seat, then they really don't have a 'saver' program," says Mr. Bruya, who ended up paying United 360,000 miles for two business-class tickets.

Asked about Mr. Bruya's frustration, a spokeswoman for United suggested that customers can buy a coach ticket for trips like his and try to use miles to upgrade to business class.

Frank Purno of Atlanta is on a waiting list for business-class seats at Delta's "SkySaver" level for a trip from Atlanta to Rome with his wife scheduled for a November departure. He's willing to pay 90,000 miles for each seat, but not the 250,000 miles per seat Delta wants for unrestricted business class.

"You would think eight months in advance I could get seats," says Mr. Purno. "It just frustrates the hell out of you."

Delta says it believes that it actually has been more generous with "saver" award availability than competitors, but that they do sell out quickly. For summer months, the airline has already booked 588 SkySaver awards in business class between Atlanta and Rome -- the route Mr. Purno wants. That's 18.1% of available business-class seats, says Jeff Robertson , managing director of Delta's SkyMiles program.

Availability does change, Mr. Robertson notes, and customers should check often for available seats. Airlines load schedules 331 days in advance -- and that's when award seats first become available. But if seats don't sell, Delta may make more of them available for awards 120 days or even 60 days before departure, he says.

"I'm absolutely convinced our SkySaver availability is better than other airlines," Mr. Robertson says.

Airlines have made more tools available to consumers to help find cheap seats. Five of the big six international carriers now have seat-availability calendars posted online that can help users spot cheap seats. Delta, the lone holdout, promises a calendar within a "few weeks." Also, some airlines give their elite-level fliers access to a greater inventory of award seats, even at the cheapest levels.

Calendars also can make it painfully apparent how little availability there is. For a Seattle-London trip checked on Feb. 22, US Airways had no seats at its lowest mileage level between April 30 and Nov. 23 -- almost a seven-month drought. Continental was better with only about a four-month absence between May 8 and Sept. 2. American actually had some 60,000-mile seats still available on that route for midweek flights in June, July and August.

US Airways says while it does offer its lowest-priced frequent flier tickets even in business class during summer months, it does have numerous blackout dates and other limits on availability.

"US Airways' strategy is to sell less discount revenue inventory in advance than other airlines," says Tom Trenga , vice president of revenue management.

One quirk of the system to remember when trying to book awards: Always check for discounted business-class and first-class tickets, which sometimes can be better values and even lower-priced than unrestricted coach tickets.