An equation-of-state model is proposed, which is based, in part, on the COSMO-RS method for molecular solvation. Free volume is incorporated into the model by consistently combining the surface contact thermodynamics with basic ideas underlying lattice-gas, lattice-fluid, or lattice-hole models. It is shown that the COSMO-RS method and Guggenheim's quasi-chemical method produce identical equations for the basic thermodynamic quantities of fluids and their mixtures. In contrast, however, with the lattice-fluid practice, the proposed model attributes a non-zero misfit energy for the interaction of an empty site with a charged surface segment. A method for the evaluation of the chemical potentials of interacting surfaces in terms of the nonrandonmess factors is proposed. The model can be applied to the liquid, gas, and supercritical states and to small molecules as well as macromolecules. Some calculations for normal alkanes and polyethylene are presented.