In this work, we use the vapor-sorption equilibrium data to show the degree of solvent upturn in each solvent-polymer system. For this purpose, 20 isothermal data sets for five polymer + solvent binaries have been used in the temperature range of 298-413 K. Solvents studied are benzene, pentane, hexane, toluene, and chlorobenzene. Homopolymers studied are: polystyrene, poly(vinyl acetate), polyisobutylene, and polyethylene. According to these data sets, solvent weight fraction in the polymer is plotted against solvent vapor activity that is calculated assuming an ideal gas phase of pure solvent vapor neglecting the vapor pressure of the polymer. We use the Flory-Huggins theory to obtain dimensionless interaction parameter, x. Also the Zimm-Lundberg clustering theory and non-ideality thermodynamic factor, F are used to interpret the equilibrium data. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 1867-1875, 2010