A direct osmosis study was carried outusing several 1:1 and 2:1 electrolytes, which diffused in a RO TFC membrane. The solute permeability D-SM/K-S/delta has been calculated using the preferential sorption capillary flow model. It was found that to some extent the ions maintain their hydration in the membrane and the permeation is a function of the hydrated radii rather than crystal ionic radii. Moreover, a linear dependence was found between electrolyte permeability through the membrane and their average diffusivity in infinite dilute solution. The relatively low permeation of 2: 1 chloride salts was attributed to the interaction with the membrane surface charge.