Adsorption equilibria of toluene, 1-propanol and their binary mixtures have been measured on Y-zeolites with three different silicon to aluminum ratios : 13, 55 and 100 at 318.15 K. The partial pressures of the pure components were varied between 0.04 and 2.20 kPa. The total pressures for the binary mixtures were adjusted to 0.35 and 1.05 kPa, respectively. The surface of zeolites usually exhibits polar and nonpolar adsorption behavior depending on the degree of dealumination. For example, highly dealuminated zeolites preferably adsorb nonpolar compounds from mixtures of adsorptives with different polarities. Therefore, in the binary mixture of toluene and 1-propanol, Y-zeolites with a Si/Al-ratio greater than 100 adsorb approximately four times more toluene than 1-propanol and show nearly ideal adsorption behavior, which can be predicted fairly well with the ideal adsorbed solution theory (IAS). At lower Si/Al-values the binary adsorption isotherms show strong nonideal behavior with an adsorption azeotropic point. This behavior-caused by intermolecular and molecule-surface interactions-cannot be predicted successfully by using the IAS-theory. Adsorbate phase activity coefficients have to be considered to take into account the real behavior. In this paper a new method for predicting these adsorbate phase activity coefficients to describe ideal and nonideal multicomponent adsorption equilibria is proposed. This model is called predictive real adsorbed component isotherms (PRAST). Based on the IAS-theory, this model includes adsorbate phase activity coefficients derived from pure isotherm data. It will be shown that real mixture behavior, caused by surface heterogeneity, e.g. the molecule-surface interactions is predicted with PRAST.