Polymers of intrinsic microporosity (PIMs) have exceptional gas separation performance for a broad range of applications. However, PIMs are highly susceptible to physical aging, which drastically reduces their long-term performance over time. In this work, we leverage complementary experimental and density functional theory (DFT) studies to decipher the inter-/intrachain changes that occur during aging of the prototypical PIM-1 and its rigidified analogue PIM-C1. By elucidating this hereto unexplored aging behavior, we reveal that the dramatic decrease in gas permeability of PIM materials during aging stems from a loss of fractional free volume (FFV) due to PIM chain relaxations induced by π-πinteractions, hydrogen bonding, or van der Waals' forces. While the PIM-1 based membranes displayed enhanced gas pair selectivities after aging, the PIM-C1 based membranes showed an opposite trend with unexpected reductions for CO2/N2 and CO2/CH4. This is due to the reductions in CO2/N2 and CO2/CH4 solubility (S) selectivities and, unlike PIM-1, the spirobisindane locked PIM-C1 (i.e., maintenance of micropore sizes) has a stable diffusivity (D) selectivities that cannot offset such reductions. These fundamental insights into the intrinsic relaxation of different PIM polymer chains during physical aging can guide the future design of high-performance PIM materials with enhanced anti-aging properties.