Byrnes and shortstop prospect Omar Quintanilla were sent to the Rockies for left-handed pitcher Joe Kennedy and right-handed pitcher Jay Witasick.
Earlier in the day, the A's finalized a trade that sent reliever Chad Bradford to the Boston Red Sox for outfielder Jay Payton. The deal had been in the works since last Thursday.
Byrnes, who was told the news before the A's optional workout at the Coliseum, had a mix of emotions as he packed his bags inside the A's clubhouse.
"I'm excited; I really am," Byrnes said. "Hopefully, I'll get an opportunity to play on a regular basis. But I have to admit I'm also very sad."
Byrnes, who grew up a Giants fan and attended St. Francis High in Mountain View, was drafted by the A's in 1998. It's the only organization he's ever played for, and he quickly developed a unique bond with the fans even before he was a regular player.
"I can't begin to thank the fans enough," Byrnes added. "I can't tell you how grateful I am for the way I was received. They've meant a lot to me. It's tough in this day and age, with free agency and trades, there's a growing disconnect between fans and players. But that's not at all what I experienced. It's helped me so much as a baseball player and as a person.
"It's been awesome playing in my hometown in front of my friends and family. It's been a realspecial experience."
Byrnes received some well wishes from the few A's players who attended Wednesday's optional workout. His eyes lit up when he thought about playing at hitter-friendly Coors Field and making three trips every year to San Francisco and San Diego.
"I don't know what to expect," he said. "But I'm going with a smile on my face and looking to help them win some games."
Byrnes was the subject of trade rumors all offseason including San Diego, Arizona and the Mets and had a feeling that this time he was finally going to be traded after the Payton news broke and Mark Kotsay signed a three-year contract on Saturday.
"It's probably just a perception we were trying to trade him," assistant general manager David Forst said. "We were not trying to trade him. He had a lot of interest, but there wasn't a deal that made sense. Colorado called, and the fit was finally there."
Byrnes entered the season as the A's starting left fielder, but he got off to a slow start, especially against right-handed pitchers, and often found himself on the bench behind corner outfielders Bobby Kielty and Nick Swisher.
"It's been frustrating," Byrnes acknowledged. "Just because I enjoy the competition. I love to play this game and be on the field. The A's made a decision they felt would help them. I can't blame them a bit."
The A's have scouted Kennedy heavily this season, and there wasn't much to see. He's 4-8 with a 7.04 ERA, giving up 128 hits in 92 innings. He was recently moved to the bullpen, and that will be his initial role with the A's, although Forst left open the possibility of Kennedy pitching as a starter in the future. The A's will control Kennedy for two more years.
"Kennedy is only 26, and he's had a lot of success in the past," Forst said. "He was always tough on us in Tampa. He still has a lot of upside."
Kennedy told The Denver Post, "Once I got put in the bullpen, I was hoping something would happen. This is a nice surprise. I'm looking forward to going to a contender."
Witasick was traded to the A's in 1996 in the Todd Stottlemyre deal and reached the majors later that year. Witasick has always been known as a pitcher with a great arm, but this will be his ninth team in 10 years (including two stops in San Diego and Oakland).
This year is one of Witasick's best, compiling a 2.52 ERA and 40 strikeouts in 35 innings, despite pitching at Coors Field.
Payton, 34, was hitting .265 with five home runs and 21 RBIs in 133 at-bats for the Red Sox. He was hitting better recently, .333 in his last 27 games, but grew increasingly frustrated with his lack of playing time.
It boiled over July 6, when Payton got into an argument in the dugout with Boston manager Terry Francona over a double switch that deprived Payton an at-bat. Payton was designated for assignment the next day, and the framework of the trade to the A's was put into place the same day.
Payton's best season came in 2003 with the Colorado Rockies, when he hit 28 home runs, drove in 89 runs and had a .512 slugging percentage in 600 at-bats.
The A's have always liked him and considered signing him two off-seasons ago, but he signed with San Diego instead. He struggled at pitcher-friendly Petco Park in 2004, hitting .260 with eight home runs and 55 RBIs, and his slugging percentage slipped to .367.
"This guy (Payton) hit 28 home runs two years ago," A's manager Ken Macha said. "He can hit righties or lefties. I remember him from the Eastern League. He was the best in the league, and Nomar (Garciaparra) was in that league."
Quintanilla is a bright prospect who impressed the A's in spring training and could be in the majors by next season. But the A's have depth with their middle infielders in the minors, including Mike Rouse at Triple-A Sacramento, Kevin Melillo at Single-A Stockton and 2005 first-round pick Cliff Pennington.