A series of univalent cation forms of zeolite Rho (M9.8Al9.8Si38.2O96, M = H, Li, Na, K, NH4, Cs) and ultrastabilized zeolite Rho (US-Rho) have been prepared. Their CO2 adsorption behavior has been measured at 298 K and up to I bar and related to the structures of the dehydrated forms determined by Rietveld refinement and, for H-Rho and US-Rho, by solid state NMR. Additionally, CO2 adsorption properties of the H-form of the silicoalumino-phosphate with the RHO topology and univalent cation forms of the zeolite ZK-5 were measured for comparison. The highest uptakes at 0.1 bar, 298 K for both Rho and ZK-5 were obtained on the Li-forms (Li-Rho, 3.4 mmol g(-1) Li-ZK-5, 4.7 mmol g(-1)). H- and US-Rho had relatively low uptakes under these conditions: extra-framework Al species do not interact strongly with CO2. Forms of zeolite Rho in which cations occupy window sites between alpha-cages show hysteresis in their CO2 isotherms, the magnitude of which (Na+,NH4+ < K+ < Cs+) correlates with the tendency for cations to occupy double eight-membered ring sites rather than single eight-membered ring sites. Hysteresis is not observed for zeolites where cations do not occupy the intercage windows. In situ synchrotron X-ray diffraction of the CO2 adsorption on Na-Rho at 298 K identifies the adsorption sites. The framework structure of Na-Rho "breathes" as CO2 is adsorbed and desorbed and its desorption kinetics from Na-Rho at 308 K have been quantified by the Zero Length Column chromatographic technique. Na-Rho shows much higher CO2/C2H6 selectivity than Na-ZK-5, as determined by single component adsorption, indicating that whereas CO2 can diffuse readily through windows containing Na+ cations, ethane cannot.