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The adsorption of CO2 on zeolite Li-Rho (unit cell composition Li9.8Al9.8Si38.2O96) has been investigated by the measurement of adsorption isotherms (273–300 K), breakthrough curves with a CO2/CH4/He mixture (308 K), and in situ synchrotron X-ray powder diffraction in CO2 (298 K). The Rho framework distorts when in the Li-form to give a shape selective adsorbent for CO2 over CH4, although breakthrough curves reveal diffusional limitations. In situ synchrotron powder XRD follows the expansion of the Li-Rho unit cell upon adsorption, which remains single phase to a CO2 pressure of ca. 0.6 bar. Partial cation exchange of Li-Rho by Na+ or Cs+ gives two series of M,Li-Rho zeolites (M = Na, Cs). Where the occupancy of window sites (8R, D8R) between lta cages is less than 50%, hysteresis is not observed in CO2 isotherms at 298 K. For Cs1.8Li8-Rho, which has a larger unit cell and a wider window than zeolite Li-Rho due to the presence of large Cs+ cations in double 8-ring sites, breakthrough curves indicate faster CO2 diffusion without significant loss of selectivity. We propose this control of adsorption kinetics of the flexible zeolite Rho via modification of cation content as a mechanism for cation controlled molecular sieving.