Flexible small-pore zeolites are interesting candidates for flue and natural gas processing due to their high sorption capacity and selectivity. It is generally accepted that their high CO2/CH4 selectivity is due to cation gating behavior such as the "trapdoor" effect or cation-controlled molecular sieving. Herein, nanosized RHO-type zeolite containing only inorganic cations (Na + and Cs +) has been prepared from a colloidal precursor suspension at 90 °C for 1 h without the use of expensive organic structure directing agents. The high Cs content significantly improved the thermal stability of the RHO nanocrystals up to 550 °C. The flexibility of the RHO cages upon water adsorption/desorption is demonstrated. The dehydration of the nanosized RHO zeolite resulted in two dehydrated RHO phases, the first one presents an enlargement of the cubic unit cell and a transformation from distorted to the more regular d8r units, and the second one presents a reduction of the cubic unit cell and an increase of the distortion of the d8r units. The 8-rings elliptical distortions of the as synthesized RHO (hydrated form) of 1.87 Å changed to 0.94 Å and 2.16 Å for the two dehydrated RHO forms. The flexibility of the nanosized RHO zeolite is due to the ability of the Cs + cation to displace out from and in to the D8R window sites which is of great importance for controlling the selective adsorption capacity of the RHO zeolite. The flexible RHO-type nanosized zeolite exhibited great selectivity towards CO2 over CH4. The adsorption capacity is retained after 10 cycles of CO2 adsorption/desorption and the crystalline structure is fully preserved.