Capping a triumphant 2006 reunion tour, legendary Brazilian rock outfit Os Mutantes will release a concert album, "Live at the Barbican," Nov. 13 via Luaka Bop. The set was taped May 22, 2006, in London, Os Mutantes' first gig since 1978.

"We rehearsed for about two-and-half months, about 12 hours a day," Os Mutantes guitarist Sergio Dias tells Billboard.com. But when the first show came, Dias says the band remained calm, despite its long absence from the stage. "We had the tingle of the challenge and we knew we were recording the gig as an historical fact, and so it was a big load of responsibility," he explains. "But I wasn't nervous."

"We had some technical problems on the first three songs but we overcame it and got ourselves together very fast. The band is pretty tight," he adds.

Dias says Os Mutantes' decision to reform stemmed largely due to demand from fans. "The fact was that kids all over the place wanted to hear us. We had no intervention from [a] record company, promotion company, marketing or anything," he says.

Backstage after the London show, the crowd chanted "Mutantes, Mutantes" for nearly 10 minutes. "After the gig, we couldn't really remember particulars of the show itself," Dias admits. "It was like entering the stage and suddenly we were at the last song."

Subsequently, the group toured the U.S., playing large clubs, festivals and culminating with a sold-out gig at New York's Lincoln Center, something Dias characterizes as "too good to be true."

Now, Os Mutantes is returning to the recording studio to lay down tracks for its first batch of new material since 1974's "Tudo Foi Feito Pelo Sol."

"Tom Ze is my writing partner now and so far it has been really wonderful," Dias reports. "We already have about seven or nine new tunes already." The plan is to introduce new songs at the Sao Paolo birthday celebration on Jan. 25, according to the guitarist.