The Flory-Orwoll-Vrij (FOV) equation is commonly used to analyze the interaction parameters of vapors in polymer melts. The theory considers the free volume effects and requires several pure component parameters, which can be determined from the pressure-volume-temperature (PVT) data of liquids. For polymers the specific volume can be predicted using three characteristic parameters over a temperature and pressure range. For low molecular weight liquids, molar core volume, V*, external degree of freedom, C, and interaction energy parameter, ET, which was introduced in this study, showed linear relationships with the carbon number for several series of homologues. A group calculation method to estimate characteristic parameters was proposed in this study. The free volume effect depended on the reduced volumes of solutes and polymers. It increased when the system temperature increased. Inverse gas chromatography (IGC) data of Flory-Huggins parameters, chi of several polar solvents in poly(vinyl chloride) and poly(tetramethylene glycol) were positive, but the exchange energy parameters, X-12, were negative and consistent with the molecular interaction consideration. (C) 2016 Elsevier B.V. All rights reserved.