The recently developed technique of dynamic light scattering from ternary solutions, in which the solvent and matrix polymer are isorefractive, can provide extensive information about the translational diffusion of polymers in semidilute and concentrated solutions. However, the strong molecular weight dependence of diffusion in these regimes has a substantial effect on the observed autocorrelation functions. It is demonstrated that the traditional cumulants approach is inadequate for extracting the desired decay rates, even for narrow distribution samples (i.e. Mw/Mn≅1.05). Alternative methods of analysis are proposed, and implications for other polymer diffusion experiments are discussed.