A new molecular thermodynamic model is developed of the gas permeability in polymer membranes on the basis of configurational entropy and Flory-Huggins theory to predict permeability dependence on the concentration of penetrant. Three kinds of configurational entropy are taken into account by this model; that is, the disorientation entropy of polymer, the mixing entropy, and specific interaction entropy of polymer/gas. The validity of the mathematical model is examined against experimental gas permeability for polymer membranes. Agreement between experimental and predicted permeability is satisfactory. (c) 2007 Wiley Periodicals, Inc.