A laboratory study using expanded granular sludge bed (EGSB) reactors was undertaken to explore the kinetic behavior of sulfate reducers and methanogens in treating sulfate-containing wastewater. A kinetic model was also formulated in a way that accounted for mass fractions of sulfate reducers and methanogens (f(SR), f(M)) and an inhibiting effect of unionized H2S on microbial groups. When the EGSB reactor was maintained at the influent COD/SO42- ratio of 0.5 (i.e., acetate was growth-limiting and sulfate was in excess), sulfate reducers kinetically prevailed for acetate over methanogens. In contrast, when the EGSB reactor was maintained at the influent COD/SO42- ratio of 0.8-3.0 (i.e., acetate was in excess and sulfate was growth-limiting), methanogens can utilize the excess acetate and may prevail over sulfate reducers. The values of f(SR), f(M) evaluated at the influent COD/SO42-ratio of 0.5-3.0 and the obtained kinetic parameter values of sulfate reducers and methanogens (k(SR), K-I,K-SR, K-sa, K-s,K-SR: k(M), K-I,K-M, K-s,K-M) together with the validated kinetic model are of help in predicting the performance of the EGSB reactor treating sulfate-containing wastewater. (C) 2011 Elsevier B.V. All rights reserved.