The adsorption of nitrogen, helium, neon, and air on Molecular Sieves 5A, 10X, and 13X was studied at 77.3 °K and in the pressure range from102to10−6Torr. The capacities of freshly activated adsorbent samples for nitrogen were found to increase markedly below10−3Torr after consecutive adsorption-desorption cycles. This “conditioning effect” may explain the large discrepancies in the data of previous investigators. The nitrogen isotherms can be represented above10−2to10−4Torr by either the Freundlich or the Dubinin–Radushkevich equations, while the helium and neon isotherms obey Henry’s law. The effect of preadsorbed water is to decrease the adsorptive capacity of Molecular Sieve 5A for nitrogen; when the amount of water exceeds 7 wt%, the capacity for nitrogen is almost completely suppressed. It is suggested that this behavior is due to the migration of hydrated cations, which block the apertures to the cube octahedral cavities of the adsorbent.