This paper presents a study of enzymatic esterification of oleic-acid and i-amyl alcohol. The product of this reaction, i-amyl-oleate, is widely used as a bio-lubricant. During the esterification reaction, water is produced as a by-product, which has a disadvantageous effect on the reaction rate and enzyme activity. To enhance the effectiveness of the process, water should be removed. One of the most promising techniques for realizing this goal is pervaporation, which can be integrated on-line with the reaction system. Such integration can be realized in the form of a membrane reactor as was studied in this work. An immobilised lipase enzyme, Novozym 435 (Novo Nordisk, Denmark), which was taken as a catalyst in the experiments, proved to be very sensitive for the presence of water and alcohol in the reaction mixture. Below a certain level of water concentration, enzymatic catalyst activity is very low. From the other side, high initial concentrations of alcohol deactivate the catalyst. Both of these influences were taken into consideration and introduced into the mathematical model of an integrated esterification-pervaporation process. Computer calculations performed with the use of this model showed that there exists a range of pervaporation process parameters which prohibits the reaction from proceeding.