The Zimm equation for solution light scattering is normally restricted to dilute solution studies and serves to characterize the solute molecular weight and radius of gyration, and its second virial coefficient describes the solute-solvent interaction. By extending our recent result obtained through use of the Flory-Huggins activity coefficient in the Einstein scattering equations, we show how a modified Zimm equation may be obtained which serves to characterize the Flory interaction parameter χAB in concentrated solutions. This approach proves useful in the analysis of neutron scattering data from polymer blends. © 1984, American Chemical Society. All rights reserved.