The prediction of multicomponent adsorption equilibria from single-component data is one of the most challenging and important problems in adsorption. The chief obstacle to progress is a scarcity of accurate and consistent experimental data over a wide range of temperature and loading for testing theories. Several binaries and one ternary system on two types of zeolites (silicalite and faujasite) were studied in a combination calorimeter-volumetric apparatus. Activity coefficients and excess functions for enthalpy, free energy, and entropy were extracted from the binary data using a three-constant equation to represent nonidealities. The successful correlation of binary excess functions with pure-component properties for type I isotherms on zeolites is a first step toward predicting multicomponent adsorption from single-gas adsorption and a major advancement over the theory of ideal adsorbed solutions.