Despite recent interest in post-polymerization modification, high-performance aromatic polymers have not received much attention as a suitable platform for the synthesis of new polymer libraries. In this article, we describe the postpolymerization modification of a polyetherimide, employing Friedel-Crafts acylation, resulting in derivatives with varied side groups and degrees of acylation. It is shown that by increasing the degree of acylation and the size of the acyl group the glass transition temperature decreases, following a linear relation, while the thermal stability is not significantly affected, and the water contact angle of the derivative membranes increases. Gas permeability tests demonstrated an increase in the permeability of the derivative membranes to N 2 and CO 2 as the degree of acylation increases. The results demonstrated the potential of the post-polymerization Friedel-Crafts acylation of highperformance aromatic polymers to obtain new and efficient membranes for gas separation.