A study of the ternary polymer systems dimethyl formamide-ethyl acetate-PS, chloroform-1,4 dioxane-PS and cyclohexane-benzene-PS was carried out by classical light scattering. Through the concept of a complex between polymer and sorbed solvent a qualitative agreement has been found between the composition light scattering and the total sorption. For the system cyclohexane-benzene-PS, it was shown that a strong and specific interaction between polymer and nonpreferentially adsorbed solvent can overcompensate the effect of the binary solvent interaction parameter chi(12). It seems, that not only the intrinsic viscosity and the coil radius of gyration but also the concentration fluctuations (or fluctuations in the structure of the polymer complex) may serve as a description of the linear expansion of the polymer coil.