Adsorption equilibria of argon, nitrogen, and methane on the 13X molecular sieve and the AS activated carbon were measured at five temperatures over a nide pressure range from 0.1 to 20 MPa using a microbalance. From experimental adsorption isotherms, which are excess functions, the absolute isotherms were calculated using equations of state for a real gas phase. Grand canonical Monte Carlo simulations and density functional theory calculations were carried out in order to explain specific features of the resulting isotherms. Different thermodynamic functions evaluated from the excess and absolute adsorption isotherms were analyzed over a wide pressure range at the average temperature of the range studied.