This study investigated the effects of factors namely temperature, palm oil mill effluent (POME) volume, inoculum volume, and co-substrate addition such as oil palm empty fruit bunch (EFB) and palm kernel on the anaerobic digestion process for biogas and methane production. Response surface methodology by the Box-Behnken design verified that the specific biogas production rate and methane yield were mainly affected by operating temperature and co-substrate addition. The optimal conditions for the maximum specific biogas production rate (0.0574 m3/ kg chemical oxygen demand per day) and methane yield (25.6) have been predicted by multiple response optimization and verified experimentally at 47.8°C operating temperature, 50.4 mL POME volume, and 5.7 g EFB addition. The error percentage between experimental and predicted values which were around 5 for methane composition and 12 for specific biogas production rate suggests the good predictability of the model. © 2011 Curtin University of Technology and John Wiley & Sons, Ltd.