Volquez pitched six effective innings, Ken Griffey Jr. hit a three-run home run and the Reds snapped an eight-game losing streak against the Astros with a 9-5 win Wednesday night.
Volquez (13-4) allowed one earned run and six hits, but the Astros chased him in the seventh with four unearned runs after two errors by shortstop Jeff Keppinger. Volquez struck out five as the Reds ended a five-game losing streak overall.
The 25-year-old right-hander hadn't won a decision since July 12 in Milwaukee, also the last time the Reds won on the road.
"I'm very happy that we're winning again," Volquez said. "Everybody was tired of losing every day."
Dunn recorded his second multihomer game of the month -- and season -- and Edwin Encarnacion added a solo shot for the Reds, who beat Houston for just the fourth time in the last 17 meetings.
Dunn has 32 homers this season, 12 this month and six since the All-Star break. On Monday, Dunn hit a grand slam off Roy Oswalt.
The slugger worked with hitting coach Brook Jacoby in late June and the minor adjustments they made are paying off.
"Me and Jake worked on a few little, I guess tweaks -- I hate that word 'tweak' -- but tweaks," Dunn said.
The Reds beat the Astros for the first time since a 4-3 victory at Minute Maid Park on May 30, 2007.
Griffey's homer was the 608th of his career, moving him within one of Sammy Sosa for fifth all-time. Griffey extended his hitting streak to 12 games and has 1,139 extra-base hits, one behind Ty Cobb for 10th all-time.
Dunn and Griffey homered in the same game for the 37th time -- but for the first time since last August.
"Any time those guys hit home runs, you've got a great chance to win," Cincinnati manager Dusty Baker said.
Wandy Rodriguez (6-4) gave up the homers to Dunn and Griffey and lost for the first time in four starts. He lasted only 4 2/3 innings after allowing one run and striking out seven in each of his previous two outings.
Dunn sent his first solo homer of the game into the upper deck in right in the second. Encarnacion then singled and scored on Joey Votto's double to make it 2-0.
Volquez gave up a single to Kaz Matsui and a walk to Lance Berkman in the first inning, then retired the next eight batters he faced. Berkman doubled to left with one out in the fourth, but Volquez struck out Carlos Lee and Geoff Blum.
Volquez was facing Houston for the first time and the Astros were impressed.
"That guy really has some explosive stuff," said Hunter Pence, who had a solo homer in the fifth. "The ball just explodes out of his hand. And even when he throws his changeup, his arm just comes at you so fast. He's got great stuff."
Volquez led off the fifth with a single and Jay Bruce beat out an infield hit. One out later, Griffey launched Rodriguez's 0-1 pitch into the upper deck in right, his 15th homer of the season.
Rodriguez hadn't allowed more than one homer in a game since his first start of the season.
"Wandy wasn't real sharp tonight," Houston manager Cecil Cooper said. "He didn't have his usual sharpness on his breaking ball and he just could not get out of the fifth inning. He left a fastball over the plate to Griffey and after that, at that point, we made it really hard on ourselves."
After Brandon Phillips walked, Dunn homered just above the yellow line in center. Dunn hit two homers in Cincinnati's 8-2 win at Milwaukee on July 12.
"Adam is swinging as good as I've seen him swing since I've been here," Baker said.
Chad Paronto relieved Rodriguez, who gave up six runs and nine hits, both season-highs. Encarnacion hit Paronto's third pitch into the seats in left, his 20th homer. It was the seventh set of back-to-back homers for the Reds this season.
The Reds added a run in the seventh off Chris Sampson.
Pence started Houston's late rally with a double to right. Keppinger committed errors on consecutive grounders, allowing Pence to score. Ty Wigginton then hit a three-run homer to left.
Mike Lincoln relieved Volquez and got Berkman on a popup to end the inning.
While happy with the win, Baker was disappointed with the messy seventh inning.
"It's a relief, but we tried to help give it back to them," he said. "We've just got to cut down on those errors."
Cincinnati scored a run in the ninth off Doug Brocail.
Reds RHP Aaron Harang, out since July 8 with a strained right forearm, threw 50 pitches before Wednesday's game. Baker said Harang would throw a bullpen session on Friday and likely start a rehab assignment on Monday. ... The Astros acquired RHP LaTroy Hawkins and cash from the New York Yankees for minor league IF Matt Cusick.