Polyarylene ether ketone (PAEK) polymers are known to be stable to hydrolysis and have desirable thermal properties; however, they are susceptible to plasticization by CO2 and hydrocarbons, leading to swelling and a decrease in performance for gas separation applications. The present invention utilizes crosslinking to improve the performance of polyarylene ether ketone polymers used to fabricate membranes for gas separation applications. In particular, the crosslinking substantially increases selectivity while maintaining commercially viable permittivity values. In addition, crosslinking the polyarylene ether ketones leads to outstanding selectivities and permeation rates that are of commercial interest.