Utilization kinetics of volatile fatty acids (acetic, propionic and butyric acids) in mesophilic (35ยฐC) methaneproducing biofilms were investigated in a continuous flow reactor. The mass transfer resistances occurring inside and outside biofilms were tested and minimized during the experiments. By monitoring the biofilm growth in terms of carbon content (mgC), the utilization rate of each fatty acid was measured per unit biomass of a balanced symbiotic bioftlm containing all concerned bacterial groups. The experiments were designed and conducted to avoid bacterial population shift in the mixed consortium. Thereby, the relations between utilization rates and concentrations of the volatile fatty acids were governed and explained by enzyme-catalyzed substrate catabolisms. The acetate utilization rate fits a Monod model quite well while consumption rates of propionic and butyric acids deviated from the two-parameter model to some degree. It was found that high propionate concentrations promoted propionate utilization but inhibited butyrate consumption. Three-parameter models were discussed to fit the experimental data better. Judged by the Monod model parameters, acetate was utilized at the fastest rate with a maximum rate of 0.098 mg HAc mg C-1 h-1 followed by butyrate (apparent maximum utilization rate of 0"0334 mg HBu mg C-1 h -1) and propionate (apparent maximum utilization rate of 0"021 mg HPr mg C-1 h-9. Butyrate utilization had the lowest apparent half velocity concentration (20 mg HBu liter-9 followed by propionate utilization (99 mg HPr liter-9 while acetate utilization had the highest half velocity concentration (160 mg HAc liter-1).