A novel non-aggressive sulfonating process for aromatic polymers is described. The process encompasses: a). dissolving the aromatic polymer in a substantially anhydrous acid selected from the group consisting of concentrated sulfuric acid, chlorosulfonic acid, and oleum, b) adding an organic solvent which is inert under the conditions of the reaction, c) adding a carboxylic anhydride, d) adding a sulfonating agent, and e) carrying out the sulfonation at a temperature below 25° C. and for a time sufficient to achieve the desired degree of sulfonation. The resultant homogeneous products of the process may preferably be used to produce membranes, for example for fuel cell applications