Polyimides are well established as gas separation membranes due to their intrinsically low free volume and correspondingly high H-2 selectivity relative to other gases such as CO2. Prior studies have established that H-2/CO2 selectivity can be improved by crosslinking polyimides with diamines differing in spacer length. In this work, we follow the evolution of macroscopic and microscopic properties of a commercial polyimide over long crosslinking times (t(x)) with 1,3-diaminopropane. According to spectroscopic analysis, the crosslinking reaction saturates after approximate to 24 h, whereas tensile, nanoindentation and stress relaxation tests reveal that the material stiffens, and possesses a long relaxation time that increases with increasing t(x). Although differential scanning calorimetry shows that the glass transition temperature decreases systematically with increasing tx, permeation studies indicate that the permeabilities of H-2 and CO2 decrease, while the H-2/CO2 selectivity increases markedly, with increasing t(x). At long t(x), the polyimide becomes impermeable to CO2, suggesting that it could be used as a barrier material.