This paper presents the theory of vapor-adsorbate equilibrium of a multicomponent mixture on microporous solids. The equilibrium relationships were obtained on the basis of the concept of micropore volume filling and the adsorbed-phase model, in which adsorbate-adsorbent interactions predominate over lateral interaction between adsorbed molecules. The predominant role of micropores is extended to the calculation of adsorbed-phase composition. The only single-gas isotherm parameters are incorporated into equations for calculation of separation factors, adsorption of components, and overall adsorption. It is shown that the separation factor in micropore space is a function of occupied micropore volume, affinity coefficients, adsorbent characteristic energy, temperature, and partial pressures. The theory has been successfully applied to published data for the adsorption of binary gas mixtures on activated carbons.