Adsorption of nitrogen, oxygen, and argon is studied in zeolite mordenite (Si/Al = 5.5) with mono-, di-, and trivalent Na+, Li+, K+, Cs+, Ca2+, Sr2+, Ba2+, La3+, and Ce3+ as extraframework cations. Adsorption capacity, selectivity, and heat of adsorption show a strong dependence on the nature as well as the extent of the extraframework cations. The cation-exchanged mordenite samples show adsorption selectivity toward nitrogen from its mixture with oxygen or argon. The electrostatic interaction between adsorbate molecules and the cations are largely responsible for the observed nitrogen selectivity of the mordenite samples because nitrogen has higher quadrupole moment than oxygen or argon. The relationships between the adsorption properties and charge densities of extraframework cations show that the interaction of adsorbate molecules depends not only on the cation type but also on the positions of these cations in the mordenite structure.