Adsorption of methane and carbon dioxide in model pores has been investigated using grand ensemble simulation. The adsorbent models were designed to imitate two heterogeneous effects that are generally regarded as being of distinct origin. The first of these, energetic heterogeneity, was modeled by generating thermally disordered surfaces that were roughened on an atomic scale. The second type of heterogeneity, structural heterogeneity, was modeled using smooth walled slit pores having a range of pore widths, and adsorption in these was examined as a function of pore width distribution. Particular emphasis has been placed on isotherms and isosteric heats of adsorption (q(st)). It is shown that the rapid decrease of q(st) with adsorbate density in the early stages of adsorption cannot be satisfactorily accounted for by energetic heterogeneity. On the other hand, plots of heat of adsorption against adsorbate density are shown to be particularly sensitive to structural heterogeneity. It is argued that the isosteric heat measurements could form the basis of a valuable method for analyzing micropore distribution.