A comparative kinetic study was carried out on the anaerobic digestion of two-phase olive mill effluent (TPOME) using three 1-dm(3) volume stirred tank reactors, one with freely suspended biomass (control), and the other two with biomass supported on polyvinyl chloride (PVC) and bentonite (aluminium silicate), respectively. The reactors were batch fed at mesophilic temperature (35degreesC) using volumes of TPOME of between 50 and 600 cm(3), corresponding to chemical oxygen demand (COD) loadings in the range of 1.02-14.22 g, respectively. The process followed first-order kinetics and the specific rate constants, K-0, were calculated. The K-0 values decreased considerably from 2.59 to 0. 14 d(-1), from 1.93 to 0.23 d(-1) and from 1.52 to 0.17d(-1) for the reactors with suspended biomass (control) and biomass immobilized on PVC and bentonite, respectively, when the COD loadings increased from 1.02 to 14.22 g; this showed an inhibition phenomenon in the three reactors studied. The values of the critical inhibitory substrate concentration (S*), theoretical kinetic constant without inhibition (K-A) and the inhibition coefficient or inhibitory parameter for each reactor (n) were determined using the Levenspiel model. (C) 2004 Society of Chemical Industry.