Story highlights "We're happy. But we're done," Mike Mills says

R.E.M. actually made their decision a few months ago

There are "indications" on the album, Mills notes

"There is a great deal of sadness, but it's really celebratory," R.E.M. bassist Mike Mills says, describing his conflicting emotions the day after he, guitarist Peter Buck and singer Michael Stipe announced they were disbanding after 31 years together.

"There is sadness because I will never play on the same stage as Peter and Michael again." Yet, Mills insists, "We're doing this for good reasons, and we end up looking back at all the fun, the joy and the incredible opportunities we had."

So why is America's biggest alternative-rock band breaking up now? "It's not because we have to or we can't stand each other or we suck," Mills contends. "We're happy. But we're done."

Mills, Buck and Stipe issued the news of their split without warning on September 21st, in a statement posted on R.E.M.'s website. "A wise man once said, 'The skill in attending a party is knowing when it's time to leave,' " Stipe wrote. "We built something extraordinary together. . . . Now we're going to walk away from it."

"It was very unexpected," says Rob Cavallo, the chairman of Warner Bros. Records, the band's label. He found out "the same morning that the press release went on their website," in a phone call from R.E.M. manager Bertis Downs. "I can't believe they're breaking up, but I understand," Cavallo says. "They're too pure, too respectful of their own thing."

R.E.M . actually made their decision a few months ago, before they met in Athens, Georgia, this summer to record three new songs for a two-CD greatest-hits package, "R.E.M., Part Lies, Part Heart, Part Truth, Part Garbage" 1982- 2011, out November 15th.

The band delayed the announcement because "a lot of people are affected by this decision in a serious way," Mills says, referring to R.E.M.'s staff and crew. "We wanted everything set up the way it should be.

"And we were excited to find three really good songs to put out as a farewell," he adds. Two of the new tracks, "Hallelujah" and "A Month of Saturdays," came from demos for R.E.M.'s last studio album, " Collapse Into Now ," released in March.

The third song, a Sixties-flavored treat with sunshine-pop brass, is called, aptly, "We All Go Back to Where We Belong." It comes out as a single October 18th.

Mills can't remember when he, Buck and Stipe began seriously talking about the end of the band. "But it was discussed on the 2008 tour," he says, and during the sessions for Collapse Into Now. The group was coming to the end of its Warner Bros. deal and chose not to tour behind that record.

There are "indications" on the album, Mills notes, citing Stipe's "lyrical content" in "All the Best." "There are some straightforward see-you-laters on that one.

"It might have been talked about in more general terms before that," Mills suggests. "We'd say things like, 'We have X number of records in the contract. By the time we finish, we'll be X number of years old. Do we still want to be out there flogging it?' "

Ironically, R.E.M.'s disappointing 2004 album, " Around the Sun ," made Mills, Stipe and Buck determined to stick around long enough to redeem themselves.

"We needed to prove, not only to our fans and critics but to ourselves, that we could still make great records," Mills says, "and we made two" -- Accelerate, released in 2008, and Collapse Into Now. "We thought, 'We've done it. Now let's do something no other band has done: Shake hands and walk away as friends.' "

R.E.M. formed in Athens in 1980 with drummer Bill Berry and issued their debut single, "Radio Free Europe," in July 1981. Through heavy touring, growing airplay and a stunning creative evolution -- from the jangling enigma of 1983's Murmur to the complex and commercial dynamics of 1987's Document and 1991's Out of Time -- R.E.M. became American rock's biggest grass-roots-success story.

The day after R.E.M. broke up, Cavallo was on the phone with Green Day's Billie Joe Armstrong: "Billie said R.E.M. were the first underground band he saw that conquered the mainstream. He was 13 years old, and it changed his perception of what rock could be."

Berry quit in 1997, after suffering a brain aneurysm on tour two years earlier. The others continued while doing side projects, which they will now pursue full-time. Stipe is a film producer and active in visual arts. Buck has a long discography as a sideman for cult heroes such as Robyn Hitchcock and the Decemberists.

R.E.M.'s 15 studio albums have sold more than 85 million copies worldwide, and the band was inducted into the Rock and Roll Hall of Fame in 2007. But Mills says R.E.M.'s greatest accomplishment was that "we conducted ourselves with as much integrity as possible. We showed people you can conduct your operation on your own terms and be successful.

"In fact," he adds, "you'll have a much better time and sleep better at night."