A detailed quasielastic light (QEL) scattering study of the polystyrene-poly(methyl methacrylate) (PS-PMMA)/benzene ternary solution with both polymers in the semidilute concentration regime was carried out. Two types of the PS-PMMA/benzene solution have been studied : one at fixed composition but different total polymer concentrations, and the other at fixed total polymer concentration but different compositions. Two diffusion modes in the intensity-intensity time correlation function of the scattered light were observed in these solutions. These two modes correspond to the eigenmodes of the diffusivity matrix that governs the time evolution of the concentration fluctuations of the polymers in the ternary solution. The concentration and composition dependencies of the relaxation rates and the amplitudes of these modes were investigated. From the relaxation rates and amplitude factors, the elements of the diffusivity matrix were determined as a function of polymer concentration and composition. By using the diffusivity data, the self-diffusion coefficients of PS and PMMA and the Flory-Huggins interaction parameter chi(F), between PS and PMMA were deduced. The concentration and composition dependencies of these quantities were studied. At fixed composition, the self-diffusion coefficients of PS and PMMA were found to decrease with increasing total polymer concentration, whereas at fixed concentration they decreased with decreasing fraction of its own kind. The value of the Flory-Huggins interaction parameter, chi(F), between PS and PMMA was found to range from 0.008 to 0.026, depending on concentration and composition. The chi(F) values obtained by our method are in good agreement with those obtained by other techniques, indicating the reliability of our method.