The hybrid process of the esterification of acetic acid with ethanol, coupled with pervaporation, was studied, and a simple, three-parameter model describing the concentration profiles in the process was built. This work suggests that the activity-based model is a better method for predicting concentration pro files than the concentration model, especially in the range of higher component concentrations in which the thermodynamic constants calculated on the basis of concentrations are not true constants. The experiments were performed using a wide range of initial molar ratios c(et)/c(a) (1:1; 2:1; 3:1; 5:1) with hydrophilic membrane PERVAP 1005 GFT and ethanol-acetic acid as the reaction mixture. Appropriate activities were calculated using the UNIFAC group contribution method.