READER: I have had Gmail for half a year, and I like it, but I am now a bit suspicious. I recently exchanged some messages with a friend from our Air Force days of 40 years ago, and I noticed that the list of "sponsored links" at the side all made reference to Air Force-related sites. Does Gmail scan message content to tailor that kind of advertising? If so, can I disable it?

A: You've had Gmail for half a year and you're just now noticing the ads alongside open messages? Guess they aren't working very well. (emphasis added--CHS)

Yes indeed, these little text ads are generated by scanning the message contents. Google, which owns Gmail, says, "The matching of ads to content is a completely automated process performed by computers. No humans read your e-mail to target the ads, and no e-mail content or other personally identifiable information is ever provided to advertisers."

If it's any consolation, e-mail services routinely scan the contents of messages to filter out junk mail and identify viruses. And no, you can't turn it off.

In June, Luke Mitchell's student marketing service, Reach Students, ran a series of Web ads to promote an offer from a major parcel delivery service. Only 0.04% of those people who got the ads on their screens bothered to click on them. He had expected at least 1% to respond.

The truth about online ads is that precious few people actually click on them. And the percentage of people who respond to common "banner ads," the ubiquitous interactive posters that run in fixed places on sites, is shrinking steadily. The so-called click-through rate for those ads on major Web destinations declined from 0.75% to 0.27% during 2006, according to Eyeblaster, a New York-based online ad serving and monitoring firm. It says that last March the average click rate on standard banner ads across the whole Web was 0.2%.

That's bad news for advertisers, who increasingly pay based on the number of banner ads served up, not the clicks they draw. Response is especially low on sites with Web-savvier audiences, such as social networking sites.

Of course, not everyone is happy with the move toward targeted ads. On Nov. 1, the Federal Trade Commission was to begin hearings related to consumer privacy and online advertising.

Marketers see increases of 30% to 300% in click rates when ads are customized based on criteria such as the location, content of Web pages visited, or information researched on search engines.