A kinetic model to describe the anaerobic digestion of olive mill wastewater (OMW) and condensation water from thermally-concentrated OMW has been developed. Two complementary kinetic descriptions, for substrate utilization and methane production, were derived. The model equations accounted for extracellular hydrolysis of complex substrates (OMW) and considered the hydrolysed products as the limiting substrates for cell growth and product formation according to Monod kinetics. Predicted relationships agreed well with experimental data. When extracellular hydrolysis was required and rate limiting, as in the case of the OMW, the equations were reduced to a Contois-type form. However, if no extracellular hydrolysis of the substrate was involved, as in the case of the condensation water from thermally-concentrated OMW, the equations were in the form of a Monod-type relationship. Thus, the equations represented a generalized kinetic expression for anaerobic fermentation relating the Monod and Contois equations, which held in the two extreme cases.