Experimental light scattering data from solutions consisting of two nearly monodisperse different molecular weight polystyrenes in toluene are interpreted using a theoretical approach recently developed in our laboratory. By allowing polystyrene chains with different chain lengths to have different monomer friction coefficients and different excluded volume parameters, we have provided an excellent theoretical model for this ternary polymer solution over a wide concentration range. The self-diffusion coefficients of two polystyrenes in various concentrations are calculated from diffusivity matrix elements. The results obtained are found to be consistent with those obtained by using different techniques.