Chris Duncan, Scott Rolen and Scott Spiezio also homered for the NL Central leaders, who extended their lead over second-place Cincinnati to 4 1/2 games. The rivals play seven times - four in Cincinnati, three in St. Louis - during the next two weeks.
Right-hander Jeff Weaver (2-2) gave his best performance since joining the Cardinals in a July 5 trade with the Angels. He allowed six hits, including Javier Valentin's solo homer, in six innings on a muggy night.
St. Louis set a season-high with 10 extra-base hits in a game that will stand as a measuring stick for both teams - the Cardinals' most lopsided victory of the season, the Reds' worst home drubbing since a 16-7 loss to the Cubs on opening day.
The two series against their closest pursuer present the Cardinals with a chance to take big steps toward their sixth division title in seven seasons. They don't play the Reds again the rest of the way.
The Reds know they have to at least keep up during the two series in order have a shot at catching the Cardinals down the stretch. Otherwise, they'll fall back into the dense pack of teams jostling for the NL wild card.
Sensing the series' importance, the Reds tried to lure fans with offers of half-price tickets and $1 hot dogs for three of the series' four games. Crowds at Great American Ball Park have been a disappointment - averaging under 27,000 - despite the Reds' midseason moves to stay in playoff contention.
The cut-rate seats and dollar dogs attracted 34,262 fans on Monday. A few hundred of them were still waiting in line for tickets outside the ballpark while the Cardinals pulled off their biggest first inning of the season - five emphatic runs that showed who's in charge.