It is generally accepted in membrane technology that crystalline zones in polymeric membranes do not contribute to transport of liquids or gases. In the current study, poly(3-alkylthiophene)s (P3ATs), i.e., homopolymers and random copolymers, were synthesized to study the influence of the supramolecular organization on membrane gas separations. The monomers were polymerized via KCTCP, and GPC analysis shows that the polymers have a narrow dispersity. DSC analysis of the polymers reveals that the homopolymers, in contrast to the copolymers, crystallized, confirming their higher degree of supramolecular organization. This was supported by UV vis absorption spectra of the polymer films, where a red shift and a characteristic shoulder absorption peak around 600 nm were observed for the homopolymers, while absent for the copolymers. More surprisingly, the homopolymers were found to be 2 orders of magnitude more permeable to CO2 than the copolymers and also more selective.