Nanofiltration membranes have been used to study their selectivity properties employing different feed solutions such as aqueous solutions of HCl, NaCl, CaCl2 and Na2SO4 in single and binary mixed systems. A marked change in the rejection of chloride due to the addition of a second solute, sometimes approaching negative values, has been observed. The rejection curves at high and low volume fluxes obtained at various operating pressures have been generated so as to cover the shifting of positive to negative rejection. Phenomenological equations based on the irreversible thermodynamics approach and Donnan equilibrium theory have been used to derive an expression for chloride rejection for a binary mixed system. The applicability of the model has been tested by comparing the computed and experimental chloride rejection data.