A simulation model of organic-matter anaerobic degradation described earlier by the authors was used for the description of sulfate influence on methanogenesis. It was calibrated and verified on the experimental data of Parkin et al. (1990), where the anaerobic chemostats were maintained at changing COD/S ratios from 60/1 to 2/1, with acetate as the carbon source. The model was applied to investigate the interaction between sulfate-reducing and methane-producing bacteria. The stimulation of the sulfate step increase showed that there was a threshold value of hydrogen sulfide concentration at which the system failed. When the COD/S ratio was less than 10/1, both sulfate reduction and methane production shut down. Hydrogen sulfide and pH inhibitions were the main factors in the model for system failure, and the former played the role of trigger stimulating the positive feed-back loop between an increase in acetate and sulfate concentrations in the reactor and a decrease in the pH level through microorganism activity. The simulations showed that there was an oscillating coexistence of MPB and SRB under the hydrogen sulfide inhibition and the pH regulating effect