A new group-contribution lattice–fluid equation of state (GCLF-EOS), which is capable of predicting the equilibrium properties of polymer–solvent solutions, was developed by modifying the original GCLF-EOS of High and Danner. The GCLF-EOS is a group-contribution form of the Panayiotou–Vera equation of state based on the lattice-hole theory. Group contributions for the interaction energy and reference volume were developed based only on the saturated vapor pressure and liquid densities of low molecular weight compounds. For a mixture, a binary interaction parameter was introduced into the mixing rules. Group contributions for the binary interaction parameter were developed from the binary vapor–liquid equilibria of low molecular weight compounds. This modified GCLF-EOS model gives excellent predictions of solvent activity coefficients both at infinite dilution and at finite concentrations. It is significantly better than the original GCLF-EOS model in its prediction capability. The only input required for the model is the structure of the molecules in terms of their functional groups. No other pure component or mixture properties of the polymer or solvent are needed.