In this era of nationally televised talent shows and test-tube-baby bands, magic is sorely lacking in pop. Yet Sérgio Dias, 55, guitarist and founding member of influential Brazilian ensemble Os Mutantes, uses that word—"magic"—repeatedly, apropos of the group's decision to re-form after three decades.

An air of the unexpected always surrounded Os Mutantes. In the late '60s, the band—Dias, brother Arnaldo Baptista, and Rita Lee—were seminal figures in the Tropicália movement, along with Caetano Veloso and Gilberto Gil. Os Mutantes fused psychedelic rock with elements of bossa nova, and the mere use of electric guitars found them branded heretics at home; the inclusion of such oddities as a can of aerosol bug spray for percussion on their eponymous 1968 debut intensified their aura of weirdness. But that experimental spirit—and recordings including "Ando Meio Desligado," "A Minha Menina," and "Baby"—also ensured their place in history, with Beck, David Byrne, and Kurt Cobain ranking among their fans.

Such endorsements were not lost on Dias, who has continued playing solo since Mutantes' final dissolution in 1978. "When you see the people who shape opinions, listening to music that you made 30 years ago, and praising it, that is when you know the music doesn't really belong to you. It has a life of its own." As to Mutantes' sustained popularity, he attributes it primarily to their youthful exuberance, and "the freedom that we had, the freshness of not being held by any preconceived ideas or A-B-A music forms. We were very, very free... and we still are."

On May 22, 2006, Mutantes performed live for the first time since 1973, at the Barbican in London, as part of a Tropicália retrospective. Yet according to Dias, at first he and Baptista were as surprised as anyone else to hear of Mutantes' involvement. "Suddenly I started to receive news that I was going to play again in London," he recalls. "Then I heard them say on the radio that me and Arnaldo were already rehearsing for the Barbican. All of this without anybody contacting us."

Initially, the brothers denied the rumors. But then they began to consider the possibility, and roped original Mutantes drummer Ronaldo "Dinho" Leme into the discussion. "Dinho is a very serious, down-to-earth person," insists Dias. "When he said, 'If they want us to play, I can play,' I knew this was the real thing. Because Dinho hadn't touched a drum in 30 years. We all came over to my place, and started playing together, and the magic was there."

Magic. That word again. Magic's the primary reason this re-formation took off, where previous ones (including a legendary 1993 offer by Cobain) never materialized. "The other times... it was somebody offering us money, or a record-company deal, which is a blah-blah-blah thing. We play because we want to play. And that's what's happening, now, again. Suddenly, the States and England are calling us."

Dias also attributes heightened excitement within the group to new addition Zélia Duncan. "When we restarted the Mutantes, we were thinking of a girl," says Dias. (Rita Lee left Mutantes less-than-amicably in 1972, and launched an extremely successful solo career; she reportedly turned down Dias's invitation to join in this reunion.) The guitarist remembered Duncan, whom he'd met and immediately liked when both of them were participating in a mutual friend's recording project.

"After Rita declined, I thought of Zélia, and I told the guys. And they said, 'Wow... but her voice is so different.' But the most important thing in a band is not the quality of the voice, in terms of texture, but the interaction between the people. Zélia is so strong and alive. When she got into the rehearsals, and started to sing, the energy of the band doubled."

That electricity manifested itself at the Barbican ("It was one of those shows where you have no recollection of it, because of the intensity and the magic," says Dias), and, one hopes, will continue through the band's handful of U.S. summer dates. A DVD and CD of that show are currently in production. And that may be only the beginning of new era for Os Mutantes. "We're starting to write with Zélia, which is great," concludes Dias. "I don't know what will happen, but hopefully there is some new material down the line. I just hope we can live up to our own standards."