We propose hydroxyl-functionalized polytriazole as a precursor for the preparation of highly crosslinked membranes and carbon molecular sieves (CMS) for gas separation. We studied the effect of the treatment temperature on the chemical structure and gas separation properties. A progressing crosslinking structure was formed when polytriazole films were treated in the range of 300–400 °C. Above 425 °C, CMSs with multi-layered nitrogen-graphene-like structures were obtained. The CO2 permeability increased by increasing the temperature, while the CO2/CH4 selectivity was maintained. Permeability increases up to 37-fold compared to the untreated polymer film were obtained, aligned with a CO2/CH4 selectivity of 75. The single-gas CO2 permeability vs. CO2/CH4 selectivity data obtained for films treated at 475 and 550 °C are among the highest reported in the literature. Moreover, the mixed gas performance of these membranes is far above previously reported CO2/CH4 data plotted as mixed-gas trade-off curves, demonstrating the potential of polytriazole materials for these applications.