Zeolite |Na₁₂|-A is a commercial adsorbent, and its CO₂-over-N₂(CH₄) selectivity can be further enhanced kinetically by replacing Na⁺ in the 8-ring windows that control gas diffusion with large cations. In this study, samples of zeolite |Na_12–xK_x|-A with x = 0.0, 0.8, 2.0, and 3.0 were prepared, and the positions of adsorbed CO₂ molecules were determined using in situ neutron powder diffraction through profile refinement. Adsorbed CO₂ molecules were located at three different sites within the large α-cavities in the zeolite structure, revealing the interaction between the adsorbed CO₂ and the host framework. The number of CO₂ molecules at each site depends on CO₂ pressure and follows site-specific CO₂ isotherms described with a Langmuir model. Most of the CO₂ molecules in zeolite |Na_12–xK_x|-A bridge two cations at neighboring 8-ring sites. These are relatively weakly physisorbed, and therefore, most of the working capacity of CO₂ adsorption is related to this site. The CO₂ molecules at the second most populated site are coordinated to a cation in the 8-ring plane. Some of them seemed to form chemical bonds with the O atoms of the framework as carbonate-like species and acted as chemisorption. The remaining minor fraction of CO₂ is directly attracted by Na⁺ at the 6-rings. The different positioning of physisorbed CO₂ and the presence of chemisorbed CO₂ was confirmed by in situ infrared spectroscopy.