A simple kinetic model was used to describe inhibitory effects of volatile fatty acids and pH in anaerobic digestion of organic matter. The model was based on the function suggested by Andrews [1], which includes an inhibition constant Ki, but the model simplifications assumed in Andrews' work were not considered. A graphical representation of the inhibition function is the basis of the methodology for the determination of the kinetic constants for the model, through a fitting with both field and laboratory data. The estimates of the kinetic constants obtained were 0.4 d−1, 0.002 g l−1 and 0.012 g l−1, for μmax, Ks and Ki, respectively. Furthermore, the model also allows the definition of stability regions for optimal operation of anaerobic digesters.