The physical adsorption of a simple gas on a heterogeneous surface is reformulated in terms of a description of the adsorbent surface as a collection of supersites of finite sizes. Formal expressions for the thermodynamic properties of an adsorbate on such a surface are given. Computer simulations of the isotherms and local densities of xenon adsorbed on stepped surfaces with several step separations are presented and analyzed in terms of the supersite formulation. It is argued that these surfaces can be divided into two types of supersites: the terraces and the narrow regions of strong adsorption at the bottom of the steps. Local isotherms are evaluated for adsorption on both types of supersites, and the changes in the isotherms with changes in the surface dimensions are discussed. Finally, the intersite interaction between atoms on the "terrace" and the "step" supersite is approximately calculated, and the effect of this interaction on the local. isotherms is presented. Future prospects for this novel approach to adsorption on a heterogeneous surface are briefly discussed.