A comprehensive finite difference model is developed and presented to predict concentration polarization (CP) in commercial spiral wound membranes. The model was developed by solving the non-conservative mass balance equation in the feed channel employing fluid flow profiles based on feed spacer mixing. Both ideal membranes with complete solute rejection characteristics and non-ideal membranes were considered. The model predicts CP for low, moderate as well as high solvent recovery up to 90%, The model was tested for numerical stability and convergence, and subsequently applied to generate CP and permeate flow loss information for a range of feed and operating conditions. The model results were verified experimentally on 1.89 x 10(-4) m(3)/s pilot-scale spiral wound reverse osmosis (RO) unit. Since flow loss in membranes is a dynamic phenomenon, it is anticipated that the model will be an appropriate choice for further work on predicting long-term. permeate flow loss from membrane fouling.