We proposed a novel method for designing CO2 permselective organosilica/polymer membranes with a dual-network structure composed of silica (first) and alkylamine-based (second) networks to control molecular sieving and CO2 adsorption properties in the membrane. Organosilica/polymer membranes were fabricated using 1,2-bis­(triethoxysilyl)­ethane (BTESE) or 1,2-bis­(triethoxyailyl)­acetylene (BTESA) as the first network, with polyethylenimine (PEI) as the second network via the sol–gel process. CO2 adsorption measurements of BTESE/PEI films were conducted via in situ Fourier transform infrared to evaluate the effects that different types of acid catalysts exert on CO2 adsorption properties. The results showed that only BTESE/PEI films prepared with a catalyst of acetic acid (HAc) display impressive chemical reactions between CO2 and amine groups, whereas the use of HCl may deactivate the amine groups. We found that the gas permeation properties of organosilica/PEI membranes were greatly dependent on the Si-precursor. Almost no selectivity could be confirmed for BTESA/PEI membranes, although pure BTESA membranes did show molecular sieving properties. However, BTESE/PEI membranes showed improved separation performance compared with that of pure BTESE membranes due to a reduction in the free volume (BTESE: H2/CH4 selectivity < 100, BTESE/PEI: H2/CH4 > 100). Moreover, the pore size of BTESE/PEI membranes could be controlled via the BTESE/PEI ratio. In conclusion, we successfully designed a dual-network structure with a controlled pore size via changes made to the Si-precursor and/or to the Si-precursor/PEI mixing ratio.