The surface force pore-flow (SFPF) model is based on solute-membrane material interactions at the membrane-solution interface. It has been used to model the transport of solute and solvent through membranes under the influence of such forces and to determine the separation efficiency based on the assumption of circular pores. It is evident from the model that the pore diameter has a strong bearing upon the separation factor and the mean separation factor can be determined from the pore size distribution. In this work, the mean separation factor (f(mean)) of a cellulose acetate membrane based on the SFPF model has been determined by the Monte Carlo simulation technique. The value of f(mean) was calculated by averaging the separation efficiency of 100 000 pores, generated at random from a Gaussian distribution. The results have been compared with the experimental reverse osmosis data for a NaCl/cellulose acetate/water system. The Monte Carlo technique is an improvement over the numerical integration scheme reported in the literature, as it considers the collective effect of all pore sizes and is not restricted by the upper and lower boundaries of numerical integration.