A mathematical model to predict the hydrolysis rate of oils by lipase, from a proposed kinetic mechanism of the reaction is derived. The model predicts interfacial saturation at high enzyme concentration. It is used to determine, under different operating conditions, the critical enzyme concentration, i.e., the enzyme concentration at which the interface between the oil and the aqueous phase containing the enzyme is saturated. To verify the model predictions, experimental results of the hydrolysis of palm oil and sunflower oil are used. The model predictions closely agree with the experimental results for sun flower oil. The sensitivity of the model predictions to the values of the kinetic parameters is also investigated. (C) 2004 Elsevier B.V. All rights reserved.