Abstract: The relationship between osmotic pressure difference across the membrane and mass transfer coefficient is developed in this paper. On the basis of this relationship, a method for measuring mass transfer coefficient by using experimental data on reverse osmosis is established. Pitzers equations are used to calculate osmotic pressure differences in order to assure accuracy of results. Under the conditions of constant operating pressure and bulk flow, mass transfer coefficient is scarcely affected by membrane structure, but decreases slightly with increasing feed concentration. The solute concentration in the polarization layer is calculated by using the measured values of mass transfer coefficient. Polarization layer concentration increases with augmentation in bulk concentration. However, their difference increases with increasing bulk concentration until a maximum difference is reached, and then decreases. Mass transfer coefficient increases with higher velocity of bulk flow. If mass transfer coefficient is so large that the ratio( P wΔ πσ/k )becomes very small, polarization could be neglected.