Kinetic constants (mu(max) and K-s) describing the predominance of lactate oxidation and fermentation were determined in chemostat cultures. The kinetics of sulfate reduction and lactate utilization were determined from 0.5 to 5 d residence times at feed sulfate concentrations of 1.0-10.0 g l(-1). The kinetics of lactate fermentation in the absence of sulfate were investigated at residence times of 0.5-5 d. The lactate oxidizers (LO) were characterized by a mu(max) of 0.2 h(-1) and K-s value of 0.6 g l(-1) compared with a mu(max) of 0.3 h(-1) and K-s of 3.3 g l(-1) for the lactate fermenters (LF). Using mathematical models, it was shown that LO competed more effectively for lactate at low lactate concentrations (<= 5 g l(-1)) and high sulfide concentrations (0.5 g l(-1)). Lactate fermenters outcompeted the oxidizers under conditions of excess lactate (>5 g l(-1)) and low sulfide (0.014-0.088 g l(-1)). (C) 2011 Elsevier Ltd. All rights reserved.