Performance of a laboratory-scale integrated baffled reactor for the treatment of raw palm oil mill effluent (POME) was investigated. Initially, the reactor was fed with diluted POME (COD=1,830 mg/L and OLR=0.46 g COD/L day) which was then increased gradually to actual concentration (COD=45,500 mg/L and OLR=11.38 g COD/L day). Reactor operation was studied in two different hydraulic retention times (HRTs) (4 and 6 days) using POME with no effluent-recycled feed and after alkalinity supplementation. Chemical oxygen demand (COD) removal of 79 and 83 at an HRT of 4 and 6 days were attained at the highest organic loading rate (OLR=11.38 g COD/L day). The presence of Arcella-like and Metopus-like species and pH profile in the bioreactor 's compartments imply that anaerobic system is active in the reactor throughout the study. Use of methanogen-enriched inocula, smooth OLR augmentation, and appropriate separation of acidogens and methanogens in the reactor were the reasons for satisfactory performances of the system. © Springer International Publishing 2014.