A lipase from Aspergillus niger immobilized by adsorption on microporous, polypropylene hollow fibers was used to effect the continuous hydrolysis of the glycerides of butter oil at 40°C and pH 7.0. The effluent concentrations of 10 different free fatty acid products were measured by highperformancee liquid chromatography (HPLC). Multiresponse nonlinear regression methods were used to fit the data to a multisubstrate rate expression derived from a Ping Pong Bi Bi mechanism in which the rate‐controlling step is deacylation of the lipase. Thermal deactivation of the enzyme was also included in the mathematical model of reactor performance. A postulated normal distribution of vmax with respect to the chain length of the fatty acid (with an additive correction for the degree of unsaturation) was tested for statistical significance. The model is useful for predicting the free fatty acid profile of the lipolyzed butteroil product over a wide range of flow rates.