Isosteric heats of adsorption of pure SF6 and CO2 were calorimetrically measured on a sample of pelletized silicalite with alumina binder. The corresponding heats of adsorption on a sample of gamma alumina were also measured. The silicalite is a homogeneous adsorbent for these gases, but the presence of the binder in the pelletized silicalite sample introduces an energetic heterogeneity for adsorption of these gases. However, the heterogeneity is largely masked for adsorption of SF6, except in the region of very high coverage because it is only weakly adsorbed on the binder. The heterogeneity for adsorption bf CO2 on the pelletized silicalite is more pronounced because it is very strongly adsorbed on the binder and the binder exhibits a high degree of heterogeneity for its adsorption. A thermodynamic model is presented for calculation of the isosteric heats of a gas on a composite adsorbent using its adsorption characteristics on the constituent materials. It is found that the slopes of the adsorption isotherms of the gas on the constituent materials play an important role in establishing the overall isosteric heat of adsorption of the gas on the composite material.