New polysilsesquioxane (PSQ)-based CO2 separation membranes with succinic anhydride and monoalkylurea units as thermally degradable CO2-philic units were prepared by the copolymerization of a 1:1 mixture of [(3-triethoxysilyl)propyl]succinic anhydride (TESPS) or [(3-triethoxysilyl)propyl]urea (TESPU) and bis(triethoxysilyl)ethane (BTESE). The succinic anhydride and monoalkylurea units underwent thermal degradation to form ester and dialkylurea units, respectively, with the liberation of small molecules (e.g., CO2 and NH3) under N2 atmosphere. The effects of thermal degradation on the performance of the obtained membranes were investigated. The TESPS-BTESE- and TESPU-BTESE-based membranes calcined at 250 ºC and 200 ºC exhibited good CO2/N2 permselectivities of 20.2 and 14.4, respectively, and low CO2 permeances of 7.7 × 10-8 and 7.9 × 10-8 mol m-2s-1Pa-1, respectively. When the membranes were further calcined at elevated temperatures of 350 ºC and 300 ºC, respectively, to promote the thermal degradation of the organic units, the CO2 permeances increased to 1.3 × 10-7 and 1.2 × 10-6 mol m-2s-1Pa-1, although the CO2/N2 permselectivities decreased to 19.5 and 8.4, respectively. These data indicate possible tuning of the CO2 separation performance of PSQ membranes by controlling the thermal degradation of the organic units.