The kinetics of the anaerobic digestion of olive mill wastewater (OMW) was studied in the mesophilic and thermophilic ranges of temperature. Two completely mixed continuous flow bioreactors operating at 35 degrees C and 55 degrees C and with an average biomass concentration of 5.45 g VSS litre(-1) were used. The thermophilic reactor worked satisfactorily between hydraulic retention times (HRT) of 10 to 40 days, removing between 94.6 and 84.4% of the initial chemical oxygen demand (COD). In contrast, the mesophilic reactor showed a marked decrease in substrate utilization and methane production at a HRT of 10 days. TVFA levels and the TVFA/alkalinity ratio were higher and close to the suggested limits for digester failure. The yield coefficient for methane production (1 CH(4) STP g(-1) COD(added)) was 28% higher in the thermophilic process than in the mesophilic one. Macroenergetic parameters, calculated using Guiot's kinetic model, gave yield coefficients for the biomass (Y) of 0.18 (mesophilic) and 0.06 g VSS g(-1) COD (thermophilic) and specific rates of substrate uptake for cell maintenance (m) of 0.12 (mesophilic) and 0.27 g COD g(-1) VSS.day(-1) (thermophilic). The experimental results showed the rate of substrate uptake (R(s); g COD g(-1) VSS.day(-1)), correlated with the concentration of biodegradable substrate (S(b); g COD litre(-1)), through an equation of the Michaelis-Menten type for the two temperatures used.