A model was developed to simulate the performance of spiral wound membrane modules for the pervaporative removal of volatile organic compounds (VOCs) from water. Differential mass and momemtum balances were solved simultaneously by numerical integration to model both feed and permeate streams. With input of the initial feed concentration, the feed side mass transfer coefficient, and physical parameters to describe the membrane and the module, the model predicted permeate pressure and velocity, the composition and average molecular weight of the permeate, the total and component fluxes, and the feed concentration, all as a function of the position in the module. Dimensionless groups that characterize the physical properties of the VOC and the design and operating parameters of the membrane and module were varied to demonstrate their effect on the overall module performance, which relates directly to process economics. The results showed that assuming the permeate pressure drop to be negligible may lead to a significant overestimation of the module performance. Permeate flow in spiral wound and hollow fiber modules is similar, and equations are provided for both.