The isosteric heats of adsorption of the components of a gas mixture are critical variables for design of adsorbers for gas separation. They can be unambiguously defined by the Gibbsian Surface Excess (GSE) model of multicomponent adsorption. These variables can be experimentally measured by multicomponent differential calorimetry (MDC) and directly used to describe nonisothermal behavior of practical adsorbers. There is no need to make simplified assumptions about the nature and size of the adsorbed phase, as required by conventional adsorption thermodynamic models, to define the isosteric heats. Pure gas isosteric heats of adsorption of N-2 and CO2 on a pelletized silicalite sample were measured using a MDC and a data analysis algorithm based on the GSE model. The silicalite sample behaved like a homogeneous adsorbent for weakly polar N-2 adsorption. The presence of polar alumina binder in the silicalite sample introduced significant heterogeneity for more polar CO2 adsorption.