The move was made Tuesday before the NL Central leaders played Colorado.
The Reds optioned Volquez to Dayton on Aug. 31 to work on his delivery and control. That opened a roster spot for Harang, activated after missing two months with lower back stiffness.
In his second start Monday since returning to the team, Harang gave up four runs, three walks and four hits in 2 1/3 innings against the Rockies.
Volquez started Monday in Dayton's final game of the season and allowed two earned runs and six hits in six innings with three walks and 10 strikeouts. In two starts for the Dragons, he had no record and a 1.38 ERA in 13 innings.
An All-Star in 2008, Volquez had reconstructive elbow surgery in August 2009 and made it back ahead of schedule, returning to the mound in July.