Petroleum refinery wastewater has the characteristic of a high concentration of aliphatic and aromatic petroleum hydrocarbons, which could lead to heavy pollution to the environment. In this study, petroleum refinery wastewater was co-treated with municipal wastewater in continuous flow bench scale reactors. Two bench scale reactors, A and B each of liquid volume of 18 L treats municipal wastewater and a mixture of municipal and refinery wastewater, respectively. The flow rate, Q for both reactors was maintained at 9 L/day with the hydraulic retention time (HRT) and sludge retention time (SRT), set at 2 days and 40 days, respectively. Food to microorganism (F/M) ratio, concentration of effluent soluble chemical oxygen demand (sCOD), mixed liquor suspended solids (MLSS), and mixed liquor volatile suspended solids (MLVSS) were monitored throughout the study period. The result of a full five months operation show high removal efficiencies of organic matter of about 90 as sCOD with 30 loading of the refinery wastewater. © 2011 WIT Press.