Rangers slugger Mark Teixeira provided the big blow, belting a three-run homer to cap Texas' four-run fourth inning. The loss was the A's seventh in 11 games, dropping them two back of the first-place Angels in the American League West.
"When you're in a pennant race," said A's manager Ken Macha, "every loss is tough."
Teixeira's homer was his career-high 39th of the year, and he took over the American League lead in RBIs with 125. The Rangers first baseman homered in each game of the series.
"They've got the big guy in the middle of their lineup," Macha said, "and he certainly put on a show against us this weekend."
Kennedy (7-11) also gave up a solo homer to Mark DeRosa in the third inning. Marco Scutaro's two-run double in the top of the fifth gave the A's a 2-1 lead, but an RBI single by Michael Young in the bottom half tied it, and Teixeira, a switch-hitter, followed with the dagger.
"You've got to stay away from the big inning here," Kennedy said. "Unfortunately, I wasn't able to do that in the fifth."
The A's trimmed the lead on Scott Hatteberg's RBI single in the top of the sixth, but the Rangers got that run back and chased Kennedy by answering in their half with an RBI double by Rod Barajas.
"I thought he threw the ball well," Macha said of Kennedy. "He left a couple of balls over the plate and paid for it."
Alfonso Soriano padded the lead with a solo homer to right off Keiichi Yabu in the seventh inning, helping Texas starter Kenny Rogers (13-7), who allowed three runs on seven hits over six innings, to pick up the win.
Mychael Urban is a national writer for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.