Modification of polyethersulfone (PES) by sulfonation with chlorosulfonic acid as sulfonating agent and concentrated sulfuric acid as solvent was carried out. Factors affecting the sulfonation reaction were studied, and reaction conditions for the preparation of sulfonated PES (SPES) with different degrees of sulfonation were determined. The presence of sulfonic acid groups in SPES was confirmed by FTIR and NMR spectroscopy. Thermal analysis of the SPES samples revealed higher glass transition temperatures and lower decomposition temperatures with respect to the PES.