A study of the effect of organic loading rate on the performance of anaerobic digestion of two-phase olive mill solid residue (OMSR) was carried out in a laboratory-scale completely stirred tank reactor. The reactor was operated at an influent substrate concentration of 162 g chemical oxygen demand (COD) dm(-3). The organic loading rate (OLR) varied between 0.8 and 11.0 g COD dm(-3) d(-1). COD removal efficiency decreased from 97.0% to 82.6% when the OLR increased from 0.8 to 8.3 g COD dm(-3) d(-1). It was found that OLRs higher than 9.2 g COD dm(-3) d(-1) favoured process failure, decreasing pH, COD removal efficiency and methane production rates (Q(M)). Empirical equations described the effect of OLR on the process stability and the effect of soluble organic matter concentration on the total volatile fatty acids (TVFA)/total alkalinity (TAlk) ratio (rho). The results obtained demonstrated that rates of substrate uptake were correlated with concentration of biodegradable COD, through an equation of the Michaelis-Menten type. The kinetic equation obtained was used to simulate the anaerobic digestion process of this residue and to obtain the theoretical COD degradation rates in the reactor. The small deviations obtained (equal to or lower than 10%) between values calculated through the model and experimental values suggest that the proposed model predicts the behaviour of the reactor accurately. (c) 2007 Society of Chemical Industry