Ozonation pretreatment was applied to palm oil mill effluent (POME) prior to anaerobic digestion using the anaerobic sequencing batch reactor (ASBR). Ozonation increased BOD/COD by 37.9% with a COD loss of only 3.3%. At organic loads of 6.48-12.96 kg COD/m(3)/d, feeding with non-ozonated POME caused a system failure. The ozonated POME gave significantly higher TCOD removal at loadings 6.52 and 9.04 kg COD/m(3)/d but failed to sustain the operation at loading 11.67 kg COD/m(3)/d. Effects of cycle time (CT) and hydraulic retention time (HRT) were determined using quadratic regression model. The generated response surface and contour plot showed that at this high load conditions (6.52-11.67 kg COD/m(3)/d), longer HRT and shorter CT gave the ASBR higher organic removal efficiency and methane yield. The model was able to satisfactorily describe the relationship of these two key operating parameters. (C) 2010 Elsevier Ltd. All rights reserved.