Our recent studies on linear-polymer blends are extended to include the linear/ring and ring/ring polymer systems. By means of the free energies of mixing, ring-polymer blends are found to be the most deviating from the Flory-Huggins theory presenting chi-parameters (interaction parameters) that depend more strongly on concentrations and molecular weights. The free energies are found to be smaller in the case of the ring-polymer blends, i.e., these blends are predicted to be the most stable, in accord with cloud-point experiments. The related neutron scattering chi(eff) parameters are found to present higher concentration dependence than the chi-parameters of these systems. By means of their mean-square radii of gyration, both ring and linear chains are found to contract when a binary polymer system approaches the phase separation limit. Close to the spinodal, the contraction is relatively stronger for the chains of the smaller concentrations and lengths, while the three systems obtain an ideal behavior at the large molecular weight, N-->infinity, limit. (C) 1988 American Institute of Physics.