Cellulose acetate membranes have been characterized in terms of pure water permeability, solute transport parameter, mass transfer coefficient, and percent solute separation with a reference system of aqueous sodium chloride solution. These membranes were used in the determination of reverse osmosis characteristics such as product rates and solute separation of zinc salt solutions. The importance of the chemical species present in solution and its effect on the degree of separation of the zinc salt has been illustrated. Correlation between the percent solute separation and the equilibrium constant for the formation of ion-pairs in solution has been established.