The adsorption of nitrogen, oxygen, and argon has been studied in Mn(Il)-cation-exchanged zeolite A and X powders and pellets at 288.2 and 303 K. Experimentally measured adsorption isotherms are compared with theoretically calculated data using grand canonical Monte Carlo (GCMC) simulation. Nitrogen showed higher adsorption capacity and selectivity than oxygen and argon in these zeolite samples. Mn(II)-exchanged zeolite A (alpha O-2/Ar = 1.2-1.4) and X (alpha O-2/Ar = 1.4-1.8) showed small selectivity for oxygen over argon. In zeolite A, the adsorption capacity for nitrogen, oxygen, and argon increases with the increase in Mn(II) exchange levels. In the case of zeolite X, the adsorption capacity for nitrogen increases at lower adsorption pressures upon Mn(11) cation exchange, but at higher equilibrium pressures, the nitrogen adsorption capacity is slightly lower than that of NaX in the granular form. However, Mn(Il)-exchanged zeolite X powder showed increased adsorption capacity for nitrogen, oxygen, and argon with increase in Mn(II) exchange levels, indicating that the binder in zeolite X affects the adsorption capacities of these gases. Isosteric heat of adsorption data showed stronger interactions of nitrogen molecules with Mn(II) cations in zeolite samples. These observations have been explained in terms of higher electrostatic interaction of nitrogen with extra-framework zeolite cations. The selectivity of oxygen over argon is explained in terms of its higher interaction with Mn-exchanged zeolites than argon molecules. This is reflected in isosteric heat data of oxygen in Mn(Il)-exchanged zeolite A (15.1 15.4 kJ/mol for O-2 and 13.3-14.3 kJ/mol for Ar) and Mn X (15.8- 17.2 kJ/mol for O-2 and 13.9-15.2 kJ/mol for Ar). Adsorption isotherms and heats of adsorption were also calculated using grand canonical Monte Carlo simulation algorithm. Simulation studies expectedly show the proximity of nitrogen molecules to the locations of extra-framework sodium and manganese cations.