The dynamic behavior of the mixture polystyrene (PS)-poly(methyl methacrylate) (PMMA)-dioxane in the semidilute concentration regime has been investigated by quasielastic light scattering. Two types of PS-PMMA-dioxane solution have been studied : One at a fixed composition but with different total polymer concentrations, and the other at a fixed total polymer concentration but with different compositions. For each solution, two distinct modes have been observed in the autocorrelation function of the scattered light; their relaxation times and relative amplitudes are studied as functions of composition and total polymer concentration. The present method differ from the previous one by not requiring one Polymer to be isorefractive with solvent. By using the experimental results and with the help of theory, the self-diffusion coefficients of PS and PMMA. and the Flory-Huggins interaction parameter chi(F) have been deduced. The chi F values obtained in this work are in good agreement with that obtained by other methods.