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A new approach to describe the adsorption behavior of liquid mixtures on solid surfaces by use of GE-models is presented. In contrast to the classical formulation of adsorption equilibrium the adsorbed phase is considered to be a mixture containing the adsorbed species (adsorbate) and the adsorbent as additional component. By introducing the Gibbs excess energy GE* for this adsorbate-solid-solution the free energy of adsorption is correlated with the respective activity coefficients of all involved components. This concept, in the following briefly called ASST (adsorbate-solid-solution theory), thus leads to a thermodynamically consistent description of both the adsorption equilibrium and caloric properties.