Polymer crosslinking via thermal oxidation is a well-established technique that enhances the gas selectivity of a polymer membrane, but usually at the expense of lower gas permeability. As thermal oxidation is typically performed at temperatures higher than the glass transition temperature (T-g) of polymers, large energy footprints are incurred while pore structures in asymmetric membranes - the preferred physical configuration of polymer membranes are collapsed. To overcome these drawbacks, here we report a strategy to simultaneously enhance both gas permeability and selectivity via thermal oxidation at sub-T-g temperatures. This was achieved with a copolymer consisting a component that enhanced T-g, and another component with functional groups that decomposed and induced crosslinking at lower temperatures. We used a series of complementary characterization techniques to pinpoint the actual mechanism underpinning our sub-T-g thermal oxidation approach. Crosslinking slightly reduced the d-space between polymer chains but increased polymer fractional free volume and most importantly, inhibited CO2-induced plasticization. The pure CO2 permeability of the crosslinked polymer reached 88.5 Barrer with a CO2/CH4 ideal selectivity of 38.8, while no plasticization behavior was detected at a CO2 pressure up to 30 atm at 35 degrees C. When separating CO2:CH4 (1:1) mixed gas at total pressures between 10 and 60 atm, the CO2 permeability and CO2/CH4 selectivity of this crosslinked polymer were reduced to 55 Barrer and 29 due to competitive permeation.