Adsorption isotherms for nitrogen, oxygen, and water, binary nitrogen-oxygen equilibrium, and ternary nitrogen-oxygen-water equilibrium were simulated in Na-X zeolite using the continuous fractional component Monte Carlo method. The effect of three different partial pressures of water on nitrogen-oxygen separation performance is evaluated, and the nature of the inhibitation of separation is examined. Even with very low water partial pressure, significant inhibition of separation and of total gas uptake is observed. The mechanism of this inhibition is shown to be a reduction of nitrogen adsorption near site-III cations.