Vinny Castilla stole the closer's role from Ugueth Urbina.
"I was just looking to hit the ball hard and make solid contact," Castilla said. "It's fun when you win, that's the bottom line."
Castilla's homer sent Montreal to its ninth loss in 11 games.
"Usually when you get those guys, it's in a big situation," Castilla said about facing Urbina. "It's special because we won the game. He's one of the toughest closers in the game."
Dave Veres (1-1) got four outs for the victory as the Rockies won their sixth straight over the Expos. Montreal put the tying run at third but pinch-hitter Robert Perez popped out to Veres.
Trailing 2-1 in the eighth, the Expos tied it when Chris Widger singled home F.P. Santangelo from third.
"We battled," Widger said. "We had a chance to win with the tying run on in the ninth. We didn't get him in but we had chances and we put ourselves in a position to win. We just didn't get the big hits when we needed them."
Colorado had gone ahead in the top of the inning against reliever Mike Maddux when pinch-hitter Jeff Reed drove in pinch-runner Curtis Goodwin from third with a one-out single past a drawn-in infield.
Rockies starter Jamey Wright allowed only an unearned run while pitching seven innings of four-hit ball. He struck out three and walked two.
"I knew I was going to have a good one," Wright said. "I just had to try not to be too perfect."
Montreal's Orlando Cabrera singled to open the third, advanced to second on a sacrifice and took third on a flyout. He scored as Terry Jones reached safely when first baseman Todd Helton dropped Wright's last-second toss to first for an error. Wright had missed attempting to tag out Jones on a slow roller to the right of the mound.
"I was two feet away and threw it right at his belt," Wright said. "It probably should have been my error, so I felt bad."
The Rockies tied it 1-1 in the fifth when Neifi Perez led off with his eighth triple and scored on Castilla's one-out sacrifice fly.
Expos starter Carl Pavano allowed one run and five hits in six innings. The rookie right-hander walked two and tied his career-high with six strikeouts, the third time he has struck out six this season.
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