Interdiffusion coefficients of the systems NH4Cl + H2O and (NH4)(2)SO4 + H2O at 25 degrees C were measured by free-diffusion optical-interferometric methods over wide ranges of concentration. Density data were also obtained for these systems. The concentration dependence of the refractive index, n, for each system was determined from refractive index increments, Delta n, the mean molar concentrations, (c) over bar, and the concentration differences, Delta c, of the set of experiments performed on each system. This dependence was compared to the concentration dependence of n predicted from equations for the molar refraction of mixtures. The latter equations use the measured concentration dependence of the solution density and either apparent molar refractions or refractive indices and densities of the pure components.