They pop up onto your computer screen while you're waiting for a Web site to load: digital intruders that are about as inviting as fingernails on a blackboard. Yet even as you curse these pop-up windows that hold you hostage while they unfurl colorful messages across a broad swath of your computer screen, you can't help but read them.

They're interstitials. Many say they're just about the most annoying thing the Net has ever dished out. But could it be that (.php!) they work?

"I personally hate them," says Dan O'Brien, a senior analyst at Forrester Research. "I think it's sort of a crude way to capture someone's attention." But he concedes that "these really annoying ads do get your attention."

You might not have even heard of interstitials. They account for only about 3 percent of U.S. Internet ad revenue, far behind banner ads' 58 percent, according to International Data Corp. But advertisers are looking for more effective ways to get their messages across. Could interstitials be the answer? "They're both more effective and more annoying" than banner ads, says Jim Nail, an online advertising analyst at Forrester.

"Interstitials" is actually an umbrella word that people use when referring to a number of related Internet advertising techniques, according to Keith Pieper, market strategist at KeithPieper.com, a Net business intelligence company. The different types of interstitials include:
> Daughter windows that open like a second browser when the user clicks on a banner.
> Pop-up windows that appear automatically when a user enters a new page.
> Splash screens that appear within the browser, reinforcing the message or introducing the site.
> Intermercials, which use rich media to entertain or provide interaction.
> Superstitials, the newest interstitial technology (created by Unicast), which load in the background in the browser's cache and play after the user has seen a Web page.

They all interrupt users' browsing experiences - and most of them slow the user's access to a desired destination. In 1997, Jupiter Communications predicted that one-quarter of all online ad dollars spent in 2001 would be on interstitials. At the current 3 percent rate, that's not likely. In a summer 1999 Jupiter survey of 3,000 consumers, 69 percent said they regarded interstitials negatively, with close to 25 percent finding them so annoying that they would avoid sites that carried them.

Mike Donahue, executive VP of the American Association of Advertising Agencies (AAAA), compares interstitials to television commercials, except for one glaring difference: Computer users, he says, are less tolerant of interruptions than TV viewers, because mouse-clickers are looking for utility, not entertainment.

Still, evidence suggests that interstitials work. When Millward Brown Interactive, the research consultancy, surveyed 7,800 Web users in July 1997 to measure the effectiveness of four interstitials, it found they were more effective than more traditional ad vehicles despite their high annoyance factor. People who see them remember the ad and the brand, and sometimes change their attitudes or what they purchase. (The study was conducted on behalf of Berkeley Systems, which develops and markets entertainment software.)

The level of recall generated by the average interstitial is more than twice that generated by a banner, says Melissa Morrison of the advertising research company Ipsos-ASI. Interstitials are eye-catching and are seen as more creative than other online ads, she says.

To increase the likelihood that interstitials will be effective, they should be used in context. "If the ad itself has an enormous amount of utility, I think people will be very tolerant," Donahue says. "It's information, not something interrupting your entertainment." For example, Women.com uses interstitials for "tune in" messages: "At 8 p.m. tonight there will be a chat with so- and-so."

This is a type of usage that Nail advocates. "I'm on the site, already involved, and you're telling me about a new feature," he says.

Pieper recommends limiting interstitials to one-quarter of the screen size to prevent problems and delays for users.

And keep them interesting, says Morrison. "It would seem that interruption and irritation can be overcome by having well-targeted, powerful creative."

The superstitials variety may be the wave of the future. A Millward Brown study released in October after a three-month test involving 2,000 Net users found them to be nearly 25 percent more effective than other kinds of interstitials - and twice as effective as banners - at creating ad awareness. Millward Brown itself recently added a superstitial to its site, offering a chance for a cash prize to everyone who'll fill out a quick survey.