This research aimed to unite virtues of intrinsically microporous polyimides (PIM-PIs) and thermally treated polymers to develop advanced high-performance membranes with not only high permeability and high selectivity, but mechanical resilience, thermal and chemical stability, and plasticization resistance. Recent research showed that thermal treatment of polymers below or above their degradation temperatures is a potential avenue for improving gas transport properties. Thermally rearranged (TR), thermally crosslinked, or carbon molecular sieve (CMS) membranes have demonstrated promising results in addressing some pressing challenges of gas separation membranes such as chemical stability and plasticization resistance. However, the thermal treatment of PIM-PIs is scarcely studied, and its effect on gas transport properties still remains vague. This Ph.D. work started by investigating the solid-state conversion of PIM-PIs into heterocyclic ring systems upon heat treatment. The PIM-PIs containing functional hydroxyl- and cyano-groups were thermally treated to derive two types of cyclisation systems – conventional polybenzoxazole (PBO) and novel isoindoloquinazolinedione (IQD). Compared to the PIM-PI derived PBO, the novel solid-state conversion of intrinsically microporous cyanoimides into IQD favorably enhanced ultramicropores with up to 80% increase in gas permeability without an expense of gas-pair selectivity. Furthermore, by studying the thermal treatment of non-functionalized and functionalized 6FDA-based polymers a long-neglected contribution of fluorine to the formation of micropores was revealed. It was concluded that the heat treatment induced a continuous fluorine release at 450 °C – crosslinking polymer chains and increasing free volume accessible for gas transport. Finally, for the first time, a hydroxyl-functionalized PIM-PI, 6FDA-HTB, was heat treated by stepwise temperature increase from below, to near and above its degradation temperature to form TR, intermediate and early-stage CMS membranes. This study provided valuable insights on the correlation between the ultra- and micropore development and gas transport properties in PIM-PIs as a function of treatment temperature. Compared to the precursor, the intermediate stage membranes possessed 20-30 times higher CO2 permeability with equivalent CO2/CH4 selectivity. Also, intermediate and early-stage carbon derivatives of 6FDA-HTB exhibited an outstanding 1:1 CO2/CH4 mixed-gas separation results well transcending the 2018 mixed-gas upper bound line. These results deemed further investigation of intermediate stage membranes attractive.