A novel pyrimidine-bridged organoalkoxysilane membrane was developed from 4,6-bis(3-(triethoxysilyl)-1-propoxy)-1,3-pyrimidine (BTPP) via a sol-gel process. Self-catalyzed and HCl-catalyzed BTPP sols with different water molar ratios were prepared for membrane formation to tailor the microstructure of the BTPP membranes. A higher water molar ratio for the HCl-catalyzed sols led to the formation of a silica network with improved porosity and a well-connected structure. Gas adsorption measurements indicated that BTPP xerogels tended to show a dense silica network due to an organic-rich hybrid structure, and these also showed a higher level of CO2/N2 selectivity due to the presence of pyrimidine groups that could conduct special interactions with CO2. Single-gas permeation testing was performed at different permeation temperatures using gases with different kinetic diameters: He (2.6 Å), H2 (2.89 Å), CO2 (3.3 Å), N2 (3.64 Å), CH4 (3.8 Å), and SF6 (5.5 Å). The BTPP membranes showed a sharp kinetic diameter dependence of gas permeance with a higher level of H2/SF6 selectivity (>500). In addition, the relatively dense silica network and organic-rich properties of BTPP membranes resulted in activated diffusion for all gases considered, with the exception of SF6 that could have permeated the BTPP membranes via larger pores or pinholes. CO2 transport behaviors through BTPP membranes were compared according to activation energies for the permeation (Ep) of CO2 and by the differences in Ep between CO2 and N2 (or CH4). The BTPP-HCl-240 membrane that demonstrated the most-improved porosity and the best-connected silica network showed a lower Ep for CO2 and a greater difference in Ep between CO2 and N2 (or CH4). As a result, the BTPP-HCl-240 membrane exhibited great potential in CO2 separation performance for both CO2 permeance and CO2/gas permselectivity. Compared with most of the reported amine-functionalized silica-based membranes, BTPP membranes showed great potential in CO2 separation performance, which could lead to applications in CO2 separation processes. [ABSTRACT FROM AUTHOR]