Marc Salem's Mind Games Tricycle, London NW6, until 8 August

To perform his act, American mind-reader Marc Salem must, by his own admission, be possessed of remarkable empathetic skills. He must know how to read people's 'non-verbal' cues, have an acute understanding of the audience's suggestibility, be certain that he can make them respond in the ways he wants them to respond.

All of which makes an incident that occurred early on the first night of his new run at London's Tricycle Theatre all the more bizarre.

Down the front, hard by the stage, was a group of kids, all girls, all aged about 10 or less. They were enjoying themselves. Salem's act demands audience participation and they were participating. Or at least they were until the point when he stopped the show, loomed over them from the stage, an imposingly rotund and bald-pated figure in shadow-black suit, as wide as he is tall, and told them to shut up.

Not in a funny way or a charming way, but like some incompetent history teacher who knows he's losing control of his class. And he didn't just lecture them once, but thrice, stepping away and then back again to hector as if the job were not yet done. The kids, on a jolly night out until that point, were stunned into silence.

At which point, he lost me entirely. The fact is that it doesn't matter how good your act is, it doesn't matter if you can predict the numbers that audience members will choose, or recite the serial number on a banknote just by feeling it, or appear to make your pulse stop - all things he can do - but if your personality is not immediately likable, the act will never fly.

Curiously, among the many ways Salem has made a living over the years - as a professor of psychology, as a consultant to the police, as a jury selector, most famously for the OJ Simpson trial - is a 10-year stint as director of research for Sesame Street. Who knows? Maybe it just put him off children for life.

To be fair to Salem (though I'm not sure he deserves it, after that episode), his act is bound to suffer from what can only be described as the Derren Brown effect. Brown's stylish and knowing brand of televised 'mentalism' has made us an increasingly sophisticated audience. Notions of non-verbal communication and suggestibility are nothing new to us, because Brown has given us chapter and verse.

He does this not because he wants us all to be home mentalists, predicting what our loved ones are going to say before they say it, but because it makes his stunts all the more impressive if he's armed us with information in advance and we still can't work them out. It also helps that, in performance, Derren Brown is intensely likable. (Who knows? Offscreen, he might be a nightmare.)

Certainly, the Derren Brown effect seemed to be playing its part with this audience. Salem could reasonably have expected the crowd to play ball with him at every turn, to allow him the reverence of a shaman, but they didn't, not entirely. In one part of his act he asks an audience member to turn on their mobile phone and dial a friend.

This evening, discovering they couldn't get a signal in the auditorium, an occupational hazard in the mentalism business these days, he asked the participant (a lawyer called Jeremy; aren't they always?) to pretend his hand was a phone, and to pretend to ask the person on the other end to choose a three-digit number.

Jeremy did as he was told, miming the phone with thumb and little finger, doing his end of the conversation before looking up at Salem and announcing, deadpan: 'She doesn't want to choose a three-digit number.' Looking at the expression on Salem's face, the entire audience suddenly became experts in non-verbal communication. An impressive mentalist he may be, but his powers as a performer are limited. When the audience did what he wanted them to do, he was fine. When they didn't, he wasn't.

For all these problems, there are still impressive moments in his act. The superb British magician David Berglas may have performed the pulse-stopping stunt on television back in the Seventies, but it was still a thrill to see it performed again here live on stage, with a dumbstruck doctor dragged up from the stalls on each arm. Pre-recording a prediction on a tape that then had to be played back to the audience was a nice twist on the usual sealed envelope device.

And the last 10 freeform minutes when he performed, with large coins tightly taped over his eyes (because the touched must always suffer for their art) and a thick blindfold over them, were engrossing. He identified objects taken from the audience without touching them. He told various individuals about the places of which they were thinking - that Jonny went to Sweden, that Dan went to Los Angeles, that Hannah was recalling an incident at St Katharine's Dock (listen to the clunk as her jaw hits the floor).

Finally, he changed the time on an unseen watch. How did he do these things? I have no idea. Because Marc Salem is a man of many talents. It is just a shame that being nice to children isn't one of them.