Anaerobic digestion of the organic food fraction of municipal solid waste (OFMSW), on its own or co-digested with primary sewage sludge (PSS), produces high quality biogas, suitable as renewable energy. We report the results from one such bench scale laboratory experiment on the co-digestion of OFMSW and PSS. The experiment used a continuously stirred tank reactor and operated at 36 degrees C for 115 days. Prior to the experiments, activity tests verified that the inoculum sludges were suitable for the biodegradation of the volatile fatty acid substrate and so producing biogas. The experimental data were used to develop and validate a two-stage mathematical model of acidogenesis and methanogenesis. In simulating the behavior of the anaerobic digestion process, including ammonia inhibition, the mathematical model successfully predicts the performance of methane production, Simulations of the pH and ammonia in the MSW anaerobic reactor were also satisfactory. Sensitivity analysis on the 18 model parameters indicated that eight of these parameters were in the most sensitive and highly sensitive range, while the remainder were in the moderate to least sensitive range. (C) 1997 Elsevier Science Ltd.