The invention disclosed relates to a process for producing azide-substituted aromatic polymers such as polysulfones, and to certain novel azide-substituted polysulfones so produced. The process involves attaching azide groups onto the aromatic rings of the polymers by first activating the attachment site by direct lithiation or bromination, followed by lithiation. The lithiated intermediates are converted substantially qantitatively to azides by reacting with a suitable azide, preferably tosyl azide, under substantially anhydrous conditions. Novel azide-substituted polysulfones containing from one to about three azide groups per repeat polymer unit were obtained, the degree of azide substitution being determined by the degree of lithiation. The azides may also be converted to other functional derivatives such as primary amines and cross-linked membranes.