A kinetic study of the anaerobic digestion process of palm oil mill effluent (POME) was carried out in a laboratory-scale up-flow anaerobic sludge fixed film (UASFF) bioreactor at mesophilic temperature (38 degrees C). Influent COD concentrations in the range of 5260-34725 mg/l were applied at steady state conditions. Chen and Hashimoto kinetic equation and a simplified Monod's model was employed to describe kinetics of POME anaerobic digestion at organic loading rates in the range of 0.88-34.73 g COD/1 day. The hydraulic retention times (HRT) ranged between 1 and 6 days. Throughout the experiment, the removal efficiency of COD was between 80.6 and 98.6%. The methane production rate was between 0.287 and 0.348 1 CH4/g COD, removed day. Biokinetic coefficients; apparent half-velocity constant, A, half-velocity constant, K-S. maximum specific microbial growth rate, mu(m), methane yield constant, Y-M, and growth yield constant, Y-x, were evaluated. The apparent rate constants, K, calculated by simplified Monod model were in the range of 2.9-7.41 CH4/g COD day. At different influent COD concentrations, K values showed a linear relationship with variations of VSS content of the reactor. (c) 2005 Elsevier Ltd. All rights reserved.