A method is proposed to show how the self-diffusion coefficients and the Flory-Huggins interaction parameter of two homopolymers in an arbitrary solvent can be determined by using quasielastic light scattering (QELS). The technique does not require one polymer to be isorefractive with solvent. The QELS data of PS/PMMA/ dioxane in the semidilute concentration regime obtained in this laboratory are used to illustrate the method. A qualitative expression to predict the critical concentration for phase separation in the ternary polymer solution is also provided.