A study of the mesophilic anaerobic digestion of two-phase olive mill effluents (TPOME) was carried out at laboratory scale. The digestion was conducted in two continuously stirred tank reactors, one with biomass immobilized on bentonite (reactor 13) and the other with suspended biomass used as control (reactor C). The reactors operated satisfactorily between hydraulic retention times (HRT) of 25 and 5 days, respectively. Soluble chemical oxygen demand (SCOD) efficiencies in the ranges of 96-80 and 93-82% were achieved in the reactors with immobilized and suspended biomass, respectively, at organic loading rates of between 0.9-4.8 and 0.9-3.8 g COD/l day, respectively. Anaerobic degradation occurred under inhibitory conditions. The overall substrate removal rate (g SCOD/l day) correlated with the biodegradable substrate concentration through the Andrews kinetic model, from which the maximum substrate removal rate (k), saturation constant (K-i) and constant of inhibition (K-i) were calculated. Although no significant differences between the k and K, values of the reactors with immobilized and suspended biomass were observed, the Ki value was 2.5-times higher in the first case, indicating that the reactor with immobilized biomass had a better response to inhibitory conditions, probably due to the increase of the sludge age. This proposed model predicted the behavior of both reactors very accurately showing deviations lower than 5% between the experimental and theoretical values of substrate removal rates. The same kinetic model allowed the anaerobic degradation of phenolic compounds to be studied, obtaining the corresponding kinetic constants. (C) 2003 Elsevier Science Ltd. All rights reserved.