Halladay (16-5) gave up Brandon Phillips' leadoff single in the first inning, then dominated the team he no-hit in the playoffs last season. His bases-loaded double in the sixth inning off Bronson Arroyo (8-11) made it 6-0.
Ryan Howard hit two of the Phillies' four homers, giving him 29 for the season. He also reached 100 RBIs for the sixth straight year, a team record.
Howard had been tied with Hall of Famer Chuck Klein for consecutive 100-RBI seasons.
Raul Ibanez returned from a strained groin and had a two-run homer among his three hits. Hunter Pence also hit a solo homer off Arroyo, who has given up the most in the majors.
The Phillies improved to 5-1 against the Reds this season and 19-6 lately. They swept their three-game division series last season, which started with Halladay throwing the second postseason no-hitter in history.
When he gets together with the Reds, memorable things tend to happen.
Any thoughts of another no-hitter were dismissed on his second pitch. Phillips singled to right field, extending his hitting streak to 13 games, the longest by a Red this season. The Reds wouldn't get another hit until Joey Votto doubled in the seventh. Halladay walked two, hit one batter and fanned nine.
Cincinnati managed four hits while getting blanked for the seventh time. Philadelphia's 18 shutouts lead the majors.
Halladay will remember this one for his swing.
He came to bat in the sixth with the bases loaded and lined a down-the-middle pitch the opposite way into the right-field corner. His awkward swing produced his first RBIs of the season and ended Arroyo's outing. The right-hander fell to 1-7 career against the Phillies.
Howard connected off Arroyo in the fourth. Pence hit a chest-high pitch for a homer that started the rally in the sixth. Arroyo has given up 36 homers, the most ever by a Reds right-hander. Left-hander Eric Milton holds the franchise record of 40 homers allowed.
Notes: Columbus Blue Jackets defenseman James Wisniewski shagged balls during batting practice and threw a ceremonial pitch. ... It was Howard's third multihomer game of the season and the 28th of his career. ... The Phillies will start LH Cliff Lee in their third game against Cincinnati on Wednesday. Lee has won his last five starts. He beat the Reds on May 26 in Philadelphia, improving to 5-2 career against them. ... Cincinnati will start left-hander Dontrelle Willis, who is still trying for that elusive first win of the season. He's 0-3 after nine starts. The bullpen has blown three save chances behind him. ... Paul Janish pinch-hit and flied out, leaving the Reds SS in an 0-for-23 rut.