In Montreal, Andre Dawson hit a three-run homer in the ninth inning to send Pittsburgh to its fifth straight loss.
Scot Thompson led off the inning with a single against reliever Cecilio Guante (4-3). One out later, Vance Law doubled to center field, sending Thompson to third, and Dawson hit a 1-0 pitch over the left field wall for his 14th homer.
"The last three times up, I swung at bad pitches," said Dawson, who homered twice Saturday. "This time I was looking for a good pitch only to get a base hit, but Guante grooved the pitch, and I hit it out. I hope I'm in the groove now. It feels good to get a big hit in the bottom of the ninth."
Tim Burke (7-0), the third Montreal pitcher, worked 1 1/3 innings to gain the victory.
Hubie Brooks tripled home two runs in the first inning and scored on Terry Francona's single to give the Expos a 3-0 lead. But the Pirates rallied to go ahead, 5-3, on Mike Brown's RBI double, Tony Pena's RBI single, Jim Morrison's RBI single, Sammy Khalifa's sacrifice fly and Marvell Wynne's homer.