Blends of an acetylene-terminated monomer (ATM) with a commercially available poly(etherimide) (PEI, Ultem(TM))were prepared, crosslinked, and characterized. Varying degrees of crosslinking were achieved through thermal treatment at 150-270 degrees C. Incorporation of the uncrosslinked additive into the PEI resulted in reductions in the glass transition temperature, gas permeabilities and selectivities, and thermal stability. These behavior are consistent with antiplasticization of the polymer host by the ATM additive. Crosslinking of the actylene-terminated additive led to increases in thermal and chemical stability and improved gas selectivities as compared to the uncrosslinked blend. Gas transport properties are reported as a function of temperature. For the blend composition considered (9 wt% ATM in PEI), the fully crosslinked blend had transport properties which were essentially equivalent to the virgin PEI. Further, processing of the blend could be achieved in the same manner as for the virgin PEI. The resistance of the crosslinked blend to chemical dissolution or swelling was markedly improved as compared to PEI.