New York ? Suspicious husbands and wives who once might have hired a private eye to find out if their spouses were cheating are now using do-it-yourself technology to check on an increasingly popular hideaway for trysts — the Internet.

Divorce lawyers and marriage counselors say Internet-abetted infidelity, romance originating in chat rooms and fueled by e-mails, is now one of the leading factors in marital breakdowns.

With the surge in cyber-affairs, a new market for electronic spying has developed. Web sites such as Chatcheaters.com and InfidelityCheck.org describe an array of surveillance products capable of tracking a cheating spouse’s e-mails and online chats, including some that can monitor each key stroke in real time.

“The traditional detective hired to chase information is being replaced by software that’s not terribly expensive but can give you 100 times the information,” said John Mayoue, a prominent divorce lawyer from Atlanta.

“It used to be that when you wanted to prove adultery, you would prove it circumstantially,” he said. “In the computer era, I can have something that is so graphic, so clear, there’s not a whole lot of room for argument.”

John LaSage, a Southern Californian, established the Chatcheaters web site after his wife of 23 years left him and their two teenage daughters without forewarning in 1999 to join a New Zealand man she had met online.

Chatcheaters — which offers advice, surveillance equipment and first-person stories of betrayal — averages 400 visitors a day, mostly women, LaSage said. His wares include $450 vehicle trackers and $100 computer-spying programs.

LaSage said he was devastated to discover, after his wife had left, that she had engaged in erotic e-mail and chat room correspondence with several men.

John LaSage demonstrates how to set a surveillance vehicle tracker underneath his truck's bumper. LaSage, a Southern Californian, established the Chatcheaters Web site after his wife left him to join a man she met online. Chatcheaters offers advice, surveillance equipment and first-person stories of betrayal. The vehicle tracker keeps a computer-readable log with exact time and locations, with the help of a satellite GPS receiver.

“I tell people to be careful — you have to be prepared for what you’re going to see,” he said.

Sandra Morris, a San Diego attorney who is president of the American Academy of Matrimonial Lawyers, said the spread of Internet infidelity has raised some complicated issues about computer privacy.

“A spouse may have a misplaced sense of entitlement to spy,” she said. “There are prohibitions against electronic eavesdropping, though a lot of people feel that when someone’s cheating, all bets are off.”

Mayoue said federal statutes outlawing interception of electronic communications can apply within a marriage.

“A spouse does have a right to privacy even from his or her own spouse,” he said. “I’ve been on both sides of this — it’s the most compelling evidence you’ll have in a divorce case, but also the most fraught with potential liability.”

David Greenfield, a West Hartford, Conn., psychologist and author of the book, “Virtual Addiction,” said many spouses who engage in cyberaffairs consider their online romances to be harmless.

“But the spouses of those who are cheating don’t see it that way,” Greenfield said. “It’s often done on the same computer they both use at home. It’s like having someone else in your own bedroom.”