The end of the summer vacation season inevitably brings a rash of serious complaints from travelers whose trips did not go as happily as they had hoped.

The staff at the luxury resort was unfriendly, or the air conditioner in their room didn't work; they found cockroaches in the tub, the luggage never arrived, or their charter flight home finally showed up two days late.

Understandably angry and disappointed, many want part (or all) of their money back, or at least an apology -- from the offending airline, an unresponsive hotel, the tour operator who put their tour package together or the travel agent who sold them the tour. A few claim that the difficulties they encountered all but ruined their entire holiday.

How do you go about getting a refund, an apology or both in such cases -- presuming your complaint is valid?

Two consumer-complaint groups can be helpful:

*The consumer affairs office of the American Society of Travel Agents (ASTA), a professional organization representing more than 20,000 travel agents, tour operators and others in the travel industry.

*The Department of Transportation's Office of Community and Consumer Affairs, which handles complaints involving U.S., foreign and charter air travel within and to and from this country.

Neither promises that you will get any money returned, though it certainly is possible. At a minimum, however, both organizations can explain what your rights are.

If you have a problem, both organizations will ask you first to try and resolve it on your own.

Step one is "don't wait until you get home" to let somebody know you're upset, advises Ray Greenly, director of ASTA's consumer affairs office. If your room is inadequate, tell the tour guide or the hotel manager. Perhaps the hotel will move you into a better one.

Of course, many problems cannot be resolved quite this simply. If you can't get satisfaction on the road, and you think the tour is not living up to what was promised in the brochure, jot down whatever details you think you will need to recall the problem when you are back home.

At home, you have two possibilities. If your trip was arranged through a travel agent, explain the problem to the agent. A good agency, wanting to keep your business, can use its influence -- and its knowledge of the travel industry -- to make an appeal on your behalf to the offending party. Again, this is assuming the travel agent agrees your complaint is legitimate.

If you didn't use a travel agent, write the hotel (tour operator, resort, etc.) directly.

It is not always easy to determine who is responsible for something that goes wrong, especially on a package tour. Package tours usually are put together by tour operators, who make the arrangements for air fare, hotels, meals and sightseeing. Travel agents sell these tours.

Usually the best party to contact first is the one from whom you purchased the tour or with whom you made the hotel or airline reservation.

But remember: There is something of an art to complaint letters. As Greenly advises, keep them brief, factual and focused on the main points of the problem. And make a statement of what you expect to receive as compensation. If you want a refund, state how much. If an apology will do, say so.

On this point, advises Greenly, "be realistic." It is doubtful you will get back the full cost of a package tour if you didn't like the hotel room you were assigned or its air conditioner temporarily broke down. Though you may think the problem ruined your vacation, the price you paid for hotel accommodations usually is only a small part of a tour package that may also include air fare, meals and guided sightseeing.

Greenly's personal opinion is that you should not use the "scatter-gun approach," sending copies of the initial complaint to every authority you can think of, including members of Congress. Instead, he suggests, keep the approach businesslike, between you and the target of the complaint.

However, if you don't hear back within a reasonable time -- from two weeks to a month -- or the response isn't satisfactory, then bring your complaint to the attention of Greenly's office. ASTA prefers that you do so in writing, sending along a copy of your initial complaint letter.

ASTA will look into your situation, says Greenly, whether or not you booked your trip through a travel agency. It will also help if your complaint is against a travel agent, whether or not the agent is an ASTA member.

Greenly's office initially will attempt "to get the other side to respond." ASTA members are required to answer such complaints or face possible loss of membership. Once both sides have presented their arguments, ASTA will act as an informal mediator, trying to obtain a mutual and satisfactory agreement. But, says Greenly, it cannot "act as judge and jury," deciding for or against anyone.

Ultimately, if an agreement cannot be reached through this process, the traveler may have to seek the advice of a lawyer. Then you have to decide whether the return will be worth the expense of legal fees.

To file a travel complaint, contact: American Society of Travel Agents, 4400 MacArthur Blvd. NW, Washington, D.C. 20007, (202) 965-7520.

Though ASTA requests that complaints be made in writing, the Department of Transportation's Office of Community and Consumer Affairs prefers that you telephone. Complaints get resolved more quickly over the phone, says spokesman Dean Witt, a DOT analyst.

DOT handles only consumer problems relating to air travel, including charter and scheduled carriers. And like ASTA, it asks the traveler to first attempt to resolve any complaint with the airline directly. If that fails, then phone DOT.

When you do, an analyst will want to know what airline your complaint is against, the date the problem occurred, where it happened, the flight number (if you recall) and what specifically you are complaining about.

The problems most easily settled are those involving aspects of air travel that are regulated by federal law, principally an airline's liability for lost or damaged luggage and proper compensation if a passenger is bumped from a flight because of overbooking.

Like ASTA, the DOT office can't force the airline to pay what you think you deserve, but it can help you get a response to your complaint. If, on the other hand, DOT doesn't think you have much of a case, you will be told that.

Many aspects of charter flights also are regulated, and these are spelled out in a contract passengers usually are required to sign. DOT may be able to help if the charter company has broken the contract by failing to provide the service as specified.

These contracts should be read carefully, says Witt. In many cases, they state that passengers could lose the full fare, sometimes hundreds of dollars, if they don't show up for the flight for any reason. If you have signed, there probably is little if anything DOT can do to assist you in getting any of the money back.

DOT also accepts complaints involving an airline's failure to offer a seat in a nonsmoking section to every passenger who requests one, another area regulated by law. If the airline appears to be lax in providing nonsmoking seats, the department "could take enforcement action," says Witt.

To file a complaint, contact: The Office of Community and Consumer Affairs, I-40, Department of Transportation, 400 7th St. SW, Washington, D.C. 20590, (202) 755-2220.