The new matchmaking site from Match.com aims to add the ineffable to the online experience, by quantifying that mysterious but essential ingredient in good relationships. That thing we call "chemistry."

Fisher uses functional MRIs to understand how being in love changes brain activity. She's the author of the popular book "Why We Love," and she believes that our romantic choices are sculpted by our experiences into what she calls "love maps."

"This is probably why it's so difficult to introduce single friends to one another and why Internet dating services often fail: matchmakers don't know the intricacies of their clients' love templates," she wrote.

To create their Chemistry.com profiles, members answer a questionnaire that includes how they see themselves and what they're looking for in a mate. They must decide whether they're performers, protectors, healers or inventors and whether they find unpredictable situations exhilarating.

They disclose whether they ever look in people's windows and whether they have interesting thoughts as they fall asleep. They choose a title for a book cover and decide whether the smile of a person in a photo is sincere.

"We're trying to take the most recent research on brain chemistry and use it to get closer to what attracts us to people," said Chemistry.com Spokeswoman Kristen Kelly.

Match.com users tend to simply want a larger pool of potential mates. Chemistry.com means to appeal to those who want more help through the whole process. While Match.com users are free to search through each others' profiles, Chemistry.com profiles are private, and each member gets introduced to just five others at a time.

In addition to the special Chemistry Profile, Chemistry.com introduced two more enhancements to the process. A step-by-step process called 1-2-3-Meet aims to get people past the "meet" stage to greeting and perhaps even actual dating.

"The true differentiator is the in-person meeting and feedback mechanism," Kelly said. "That's the piece that's been missing in this industry."

Members also get feedback after each first meeting, in the hope that they might gain personal insight that will improve their chances of mating in the long run.

"You can have chemistry online, and when you meet in person, there's nothing there," said Oshima, who runs the online dating sites DreamBachelor.com and DreamBachelorette.com, and also coaches people on how to behave online and on face-to-face dates.

Oshima said multimedia features that let people send voice or video messages were a good way to check out that face-to-face chemistry while still in the online stage.

"Somebody might be really upbeat on a voicemail message, and he would seem like a lot of fun, but that doesn't come across in e-mail," she said.

A beta test of Chemistry.com launched this week in Denver, San Diego, Seattle and Washington, D.C.. The company plans a national roll-out in early 2006.

"We all know intuitively when we have chemistry with someone, where there's a spark," Kelly said. "Dr. Fisher and other researchers now have shown there's a true physiological basis to those things we've know intuitively.

"If we can take that learning and apply it in a practical setting to help you find someone you'll have that chemistry with, how great is that?"

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