The concept of global activity coefficients is developed in terms of adsorption at the liquid/solid interface. These coefficients evaluated from the excess adsorption data characterize non-ideality of the surface phase resulting from differences in molecular interactions which are additionally perturbed by heterogeneity of the solid. A method for separating a non-ideality of the adsorbed phase caused by intermolecular forces from that generated by surface heterogeneity is presented. It is shown theoretically that the global activity coefficients plotted as functions of the bulk or surface phase composition are characteristic for a given adsorption system and are useful for assessing adsorbent heterogeneity effects. The results presented are also helpful for evaluating the surface phase capacity which is a very important quantity in the study of adsorption from solutions on solids.