Light scattering of a series of mixtures of two sharp fractions of polystyrene and polymethylmethacrylate in different proportions in toluene．and also of a sharp fraction of polystyrene in polymethyImethacrylate-toluene of different concentrations were inves-tigated．Since polymethylmethacrylate has equal refractive index as toluene，it will not contribute to the intensity of the scattered light from the solution． From the measure-ments of the light scattered at angles of 90°，45°and 135°，M>w and h2>z1/2 of polystyrene were calculated． Equal values of M>w and h2>z1/2 were obtained in all cases，while the slopes of the curves decrease steadily as the concentration of polymethyl-methacrylate in solution increased．The results indicate that the presence of polymethyl-methacrylate in the solution has no effect on (M)w and (h2)z1/2 of the polystyrene frac-tion.