A mathematical model for ethanol fermentation with yeast Saccharomyces cerevisiae and beta-galactosidase on a whey medium was developed. Kinetic parameters of biomass growth were estimated using the least squares method on the basis of experimental data obtained in batch culture of suspended yeast cells growing on glucose and in batch fermentation with immobilized enzyme and yeast cells growing on lactose. Kinetic constants of separation of ethanol by pervaporation was estimated using experimental fluxes of permeate and separation coefficients of ethanol from broth. The degree of sugars utilization and the ethanol concentration in bioreactor as well as the time of ethanol separation needed to obtain the desired final concentration of recovered product can be predicted by using this model. The model can be also used for the investigation of influence of operating conditions on the fermentation yield and on the product concentration from broth. It can help as well in the optimization of process and in planning of experiments. (C) 2007 Elsevier Ltd. All rights reserved.