Adsorbents with high capacity and selectivity for adsorption of CO2 are currently being investigated for applications in adsorption-driven separation of CO2 from flue gas. An adsorbent with a particularly high CO2-over-N2 selectivity and high capacity was tested here. Zeolite ZK-4 (Si:Al ∼ 1.3:1), which had the same structure as zeolite A (LTA), showed a high CO2 capacity of 4.85 mmol/g (273 K, 101 kPa) in its Na(+) form. When approximately 26 at. % of the extraframework cations were exchanged for K(+) (NaK-ZK-4), the material still adsorbed a large amount of CO2 (4.35 mmol/g, 273 K, 101 kPa), but the N2 uptake became negligible (<0.03 mmol/g, 273 K, 101 kPa). The majority of the CO2 was physisorbed on zeolite ZK-4 as quantified by consecutive volumetric adsorption measurements. The rate of physisorption of CO2 was fast, even for the highly selective sample. The molecular details of the sorption of CO2 were revealed as well. Computer modeling (Monte Carlo, molecular dynamics simulations, and quantum chemical calculations) allowed us to partly predict the behavior of fully K(+) exchanged zeolite K-ZK-4 upon adsorption of CO2 and N2 for Si:Al ratios up to 4:1. Zeolite K-ZK-4 with Si:Al ratios below 2.5:1 restricted the diffusion of CO2 and N2 across the cages. These simulations could not probe the delicate details of the molecular sieving of CO2 over N2. Still, this study indicates that zeolites NaK-ZK-4 and K-ZK-4 could be appealing adsorbents with high CO2 uptake (∼4 mmol/g, 101 kPa, 273 K) and a kinetically enhanced CO2-over-N2 selectivity.