We simulate the adsorption of small molecules (Xe, Ar, CH4) and their binary mixtures in zeolite NaA using the grand canonical ensemble Monte Carlo method. We report the isotherm, energy, entropy, and adsorbate density distribution. The effects of nonideal mixing in the pore are assessed by comparing the simulated binary isotherms to those predicted by a pore filling model (Ruthven, D. M. Nat. Phys. Sci. 1971, 232, 70. Ruthven, D. M.; Loughlin, K. F.; Holborow, K. A. Chem. Eng. Sci. 1973, 28, 701). At low pore loadings, we observe strong selectivity for a more polarizable molecule, but at higher pore loadings, a smaller, less polarizable molecule can adsorb selectively. This occurs at significantly lower pressures in our simulations than would be predicted by the pore filling model. This increased selectivity for the smaller molecule occurs because of differences in the two component’s ability to pack efficiently in the pore beyond that accounted for by the pore filling model.