Zeolites are mainly used as heterogeneous catalysts, with an increasing use as gas-separation adsorbents. Their properties as adsorbents are described by adsorption isotherms and isosteric heats of adsorption. Two methods are presented that predict isosteric heats of adsorption of gaseous molecules at low coverages. Both methods are based an the quantum mechanical (QM) evaluation of the interaction between the molecule adsorbed and the cationic site treated as an embedded cation. The first method uses Boltzmann statistics, whereas the other method assumes that adsorption at low pressure can be described with a Langmuir model. These two procedures yield very comparable isosteric heats for N-2 and O-2 in Ca-A and Ca-LSX. Their comparison allows us to underline the strong heterogeneity of N-2 adsorption in Ca-LSX and the effect of temperature on the equilibrium constant of adsorption.