Relation between the thermodynamic parameters obtained from water sorption isotherms and the degree of reduction in the glass transition temperature (Tg), accompanied by water sorption, was quantitatively studied. Two well-known glassy food materials namely, wheat gluten and maltodextrin were used as samples. The difference between the chemical potential of water in a solution and that of pure water (Delta G(w)(s)), the difference between the chemical potential of solid in a solution and that of a pure solid (Delta G(a)(s)), and the change in the integral Gibbs free energy (Delta G(w)(s)) were obtained by analyzing the water sorption isotherms using solution thermodynamics. The parameter Delta G(w)(s) Delta G(w)(s)s a correlated well with.T-g ( Tg -Tg0; where Tg0 is the glass transition temperature of dry material), which had been taken to be an index of plasticizing effect. This indicates that plasticizing effect of water on foods can be evaluated through the parameter Delta G(a)(s).