Solutions of polystyrene in DOP were studied as a function of the concentration (0.2-0.9 g/mL) and temperature (-10 to +130 degrees C) using static and dynamic light scattering. The concentration and temperature dependence of the cooperative diffusion coefficient and the osmotic modulus were determined. The concentration and temperature dependences of the cooperative diffusion coefficient are explained in terms of the concentration and temperature dependence of the osmotic modulus and the solvent mobility. At the concentrations studied, the effect of hydrodynamic interactions is small. Close to the glass transition temperature the solvent mobility is the dominant factor and can be treated in terms of the free volume theory.