Phase equilibria and spinodal decomposition of polystyrene/polybutadiene blends are investigated by using the Flory-Huggins approximation and the Landau expansion of the free energy. Experimental cloud point curves extrapolated to infinitely slow heating and cooling rates are adequately explained by assuming a suitable power law dependence of the effective interaction parameter X on temperature. Theoretical arguments stressing the configurational effects of a concentration gradient yield an expression for the gradient term that is substantially different from the de Gennes-Pincus formula. According to our calculations, the gradient term is a homogeneous quadratic form of the three interaction parameters of the binary mixture and has a much stronger molecular weight dependence. Finally, the interphase thickness and surface tension are evaluated near the critical point. In this limit, provided X has a power law dependence on temperature, the surface tension is found to be proportional to [(Tc-T)/Tc]3/2M-1/2g(M), where g(M) is a constant only if all of the interaction parameters have the same temperature dependence. © 1985, American Chemical Society. All rights reserved.