Pore size distribution of polyamide thin film composite reverse osmosis membranes and a cellulosic ultrafiltration membrane were obtained from vapor adsorption data of CO2and N2gases. The surface: force-pore flow model previously reported in the literature was utilized in this work. The pore distribution data were further used with solutes separation data at a particular pressure to obtain the values of solute-solvent-membrane wall forces involved. The interaction parameters were obtained by the simultaneous solution of the ordinary differential equations describing the model using the software package COLSYS. From the knowledge of the pore distribution data and the interactions forces, the solutes separation data (for nonionized organics and sodium chloride) were predicted over a wide range of pressures and showed excellent agreement with experimental separation data.