Adsorption of CO2 and N-2, both as single components and as binary mixtures, in three zeolites with identical chemical composition but differing pore structures (silicalite, ITQ-3, and ITQ-7) was studied using atomistic simulations. These three zeolites preferentially adsorb CO2 over N-2 during both single-component and mixture adsorption. The CO2/N-2 selectivities observed in the three siliceous zeolites vary strongly as the adsorbent's crystal structure changes, with the selectivity in ITQ-3 being the largest. Our studies indicate that the different electric, fields present inside zeolites with different crystal structures but identical chemical composition play an important role in the observed adsorption capacities and selectivities. The accuracy of the ideal absorbed solution theory in predicting the behavior of CO2/N-2 mixtures in silica zeolites based on single component adsorption data was also tested; this theory performs quite accurately for these adsorbed mixtures.