Isosteric heats of gas-mixture adsorption required in the energy balances of adsorption process design are calculated using a thermodynamically consistent multicomponent isotherm. This isotherm and the resulting heats of adsorption possess proper limiting behavior and account for adsorbates of different sizes. Binary and ternary mixtures are used as examples. Consideration of the temperature dependence of the adsorbed phase density or alternately of the saturation capacity leads to coupling of contributions from each component to isosteric heats of adsorption of other components in the mixture. For the binary example used, this reduces the difference between isosteric heats in the mixture and corresponding pure gas heats at the same fractional loading, in comparison to predictions where the saturation capacity is assumed to be independent of temperature.