series of univalent cation forms of zeolite Rho (M(9.8)Al(9.8)Si(38.2)O(96), M = H, Li, Na, K, NH(4), Cs) and ultrastabilized zeolite Rho (US-Rho) have been prepared. Their CO(2) adsorption behavior has been measured at 298 K and up to 1 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, CO(2) 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 CO(2). Forms of zeolite Rho in which cations occupy window sites between a-cages show hysteresis in their CO(2) isotherms, the magnitude of which (Na(+),NH(4)(+) < 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 CO(2) adsorption on Na-Rho at 298 K identifies the adsorption sites. The framework structure of Na-Rho "breathes" as CO(2) 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 CO(2)/C(2)H(6) selectivity than Na-ZK-5, as determined by single component adsorption, indicating that whereas CO(2) can diffuse readily through windows containing Na(+) cations, ethane cannot.