<p>Reducing CO₂ emissions is of primary importance in addressing global warming. Membrane separation of CO₂ has been actively studied as a straightforward and low-cost CO₂ separation method. In this study, new polysilsesquioxane (PSQ)-based membranes were prepared by the 1:1 copolymerization of (triethoxysilylethyl)benzene (TESEB), (triethoxysilylethyl)pyridine (TESEPy), and (triethoxysilylethyl)naphthalene (TESENp) with bis(triethoxysilyl)ethane (BTESE), and their CO₂ separation performance was evaluated. Among them, the TESEB-BTESE membrane exhibited the highest CO₂/N₂ permselectivity of 49.6 with a CO₂ permeance of 2.03 × 10⁻⁷mol m⁻²s⁻¹Pa⁻¹ (606 GPU). The CO₂ permeance of the TESEPy-BTESE membrane increased from 1.47 × 10⁻⁷ (439 GPU) to 3.34 × 10⁻⁷mol m⁻²s⁻¹Pa⁻¹ (997 GPU) after the membrane was stored at 200 °C for 4 weeks in a nitrogen atmosphere, although CO₂/N₂ permselectivity slightly decreased from 20.0 to 16.5. DFT calculations were performed to understand the effects of aryl substituents on the CO₂ separation performance, providing important information for the molecular design of PSQ-based membranes for CO₂ separation.</p> Graphical Abstract <p></p>