Reverse osmosis separation behavior of some metal salts of Cu2+, Ni2+, Fe3+, and A13+have been studied. The cellulose acetate membranes used were characterized in terms of pure water permeability constant, solute transport parameter, and mass transfer coefficient with a reference system of aqueous sodium chloride solution. The importance of the chemical species present in solution and its effect on the degree of separation has been illustrated. Correlation between the percent solute separation and the equilibrium constant for the formation of ion-pairs in solution has been established. Reverse osmosis treatment has been applied to an acid mine-water solution, and the suitability of this process is discussed.