An experimental design in different reaction conditions was applied to modify poly(ether imide), PEI, by sulfonation using acetyl sulfate. Higher temperatures and reaction times led to higher ion exchange capacity. The thermal analysis showed that our sulfonation approach accomplished preparing sulfonated PEI maintaining the thermal stability of its parent material even for the film with highest degree of sulfonation, and assessed the effect of this change on CO 2 , O 2 and N 2 permeabilities. The permeability results pointed out that our sulfonation approach is an effective way to produce high performance engineered polymers for fuel cell electrolyte membranes and for CO 2 separation from air. These results also suggest that the use of milder reactive medium at higher temperatures and for longer reaction times seems to be a more promising approach to achieve thermal stability than use more aggressive sulfonation agents.