To the solo traveler, the four most annoying words in the English language may be "based on double occupancy."

Although the travel industry prefers to think people travel two by two, solo travelers are not an inconsequential demographic. About 25% of all Americans who travel domestically or abroad do so alone, according to a 2009 survey by D.K. Shifflet & Associates, a Virginia-based tourism and travel research company.

Norwegian Cruise Line made waves by offering 128 "studio" suites, designed mainly for one person, on its Epic, which debuted last month. Still, the dreaded single supplement remains the norm on cruise ships, with solos paying as much as double the fare for one person if they don't share a stateroom.

As for those glossy cruise brochures trumpeting "two for one" sailings? By now I know that, in the fine print, I'll find that this does not mean a solo traveler will pay half-fare. Unless you're willing to share with a stranger, you're not likely to get a price break. My one experience, a "barefoot cruise" to the Caribbean on a windjammer, cured me. Granted, the cruise wasn't my cup of tea either, with nude beaches and beer-fueled toga parties, but my roommate, a woman from New Jersey, spent most of the voyage atop her bunk reading her Bible, convinced we were sailing straight for Sodom and Gomorrah.

Most land tours also are based on double occupancy. And, for the most part, hotels give no breaks to solo leisure travelers. Hotels don't cut their rates in half if there's only one of you.

Vendors who package tours "sell their packages per person based on double occupancy," said Tammy Weiler, president of Boca Raton, Fla.-based Singles Travel International (http://www.singlestravelinternational.com). Her company and others specializing in singles travel can generally offer two options: Pay a "single supplement" — which sometimes means paying for two people even if you're only one person — or be matched with a roommate.

Those who specialize in singles travel say solo travelers are treated like second-class citizens. And by far the most-voiced complaint is about the single supplements on cruise lines.

Epic's studio suites are "the best thing that's happened to solo travelers in decades," said Carolyn Spencer Brown, editor of CruiseCritic.com, a Web-based guide to cruising. "We hear people are thrilled." But she doesn't expect other lines will rush to follow suit, if only because staterooms on most ships now being built are prefabricated as doubles. But she thinks Norwegian's move "has awakened the industry to demand."

For the cruise lines, it's all about the bottom line: If a ship is filled to capacity, more people are spending in the bars, the casino, the premier restaurants and for shore excursions.

Kevin Sheehan, chief executive of Norwegian, said that although cruise line "is taking a bit of a financial hit," he's betting on "attracting a whole new demographic."

"The single traveler has been completely missed by this industry." He noted that although 20% of Americans have taken cruises, only 5% of single travelers have because they balk at paying for two. "We think we've got a winner here. We're sold out on a lot of our sailings." Maybe, he added, those solos "will spend more time in the bar. We're keeping our fingers crossed."

One bright note: In a poor economy, solo travelers have been able to snag some bargains. On a Holland America cruise to Alaska last summer, a friend and I each scored a double stateroom at a single fare. But, Weiler said, "as soon as the economy is back on track, [the industry] is not going to recognize us, so single travelers should take advantage of any breaks while they last."

And there are some breaks.

Tauck (http://www.tauck.com), a high-end tour operator based in Connecticut, reduced single supplements on 102 departures this year — land tours, European river tours and small-ship cruises — by an average of 42%.

Besides Norwegian, other cruise lines that offer single staterooms, including Southampton, England-based P&O (http://www.pocruises.com), whose new Azura has 17, at about 125% of the fare for one person sharing a double. The ship is in Europe this summer and will be cruising the Caribbean in the winter. One caveat: These sell out fast — and a limited number is offered to U.S. travelers.

Silversea Cruises (http://www.silversea.com), a luxury all-suite line, is offering solo fares 10% above the per-person double-occupancy fare on four cruises between now and the end of the year. The cruise line is also offering 24 voyages with a 25% single supplement between now and 2011. Most offers include economy round-trip airfare from Los Angeles. Destinations include Europe, the Mediterranean, Asia, the Middle East and South America.

Woodland Hills-based Viking River Cruises (http://www.vikingrivercruises.com) has two single staterooms on each of three ships — the refurbished Surkov, Kirov and Pakhomov, all of which will sail in Russia in 2011. Fares for a single room are half that of a double. Viking's Prestige, which will enter service next year, will have six single staterooms.

International Expeditions (http://www.ietravel.com), based in Alabama, caters to adventurous solo travelers with a series of supplement-free sailings on its small ships this year and next. These include a Sept. 1 voyage to East Greenland and 2011 cruises on the Amazon River and to the Galápagos Islands.

Generally, hotels see an unoccupied bed as a negative. "The more people in the rooms, the more people playing golf, using the spa," said Robert Mandelbaum, director of research information services for Atlanta-based PKF Hospitality Research. "Multiple occupancy is more profitable for the hotel."