The isosteric heat of adsorption is a critical design variable in estimating the performance of an adsorptive gas separation process. The heats can be strong and complex functions of adsorbate loadings when the adsorbent is energetically heterogeneous. Ignoring these characterisitics in process design can lead to serious errors. Only calorimetric heat measurements (pure and multi-component gas) can reveal the complex nature of the adsorbent heterogeneity. Examples of calorimetrically measured heats for adsorption of pure SF6 and CO2 on a silicalite sample bonded with alumina, and those for binary CO2-C2H6 mixtures on NaX zeclite are cited to demonstrate the complexity of the subject. The loading dependence of the binary heats is found to be counter-intuitive. A simple analytic thermodynamic model of patchwise heterogeneity is proposed to describe the isosteric heat of adsorption of a single gas and those for the components of a binary gas mixture. (c) 2005 Elsevier B.V. All rights reserved.