We present a new method to determine the Flory-Huggins polymer-polymer interaction parameter and self- (or tracer) diffusion coefficients in a ternary polymer solution made of two monodisperse polymers and a solvent by quasielastic light scattering (QELS). Previously proposed methods based on the random phase approximation (RPA) were restricted to special cases with two polymers having the same molecular weight and identical interaction parameters with the solvent. The present method (also using RPA) has removed these restrictions. It is applicable to an arbitrary ternary solution with polymers having different molecular weight and solvent interaction parameters. To illustrate, we have analyzed the published QELS data of the PDMS/PMMA/toluene ternary system to obtain self diffusion coefficients for each polymer and the Flory-Huggins polymer-polymer interaction parameter as a function of polymer composition in the system.