As a novel class of proton exchange membrane (PEM) materials for use in fuel cells, sulfonated poly(phthalazinones) (SPPs), including sulfonated poly(phthalazinone ether sulfones) (SPPESs), sulfonated poly(phthalazinone ether ketones) (SPPEKs) and sulfonated poly(phthalazinone ether sulfone ketones) (SPPESKs), were prepared by modification of corresponding poly(phthalazinones) (PPs). Sulfonation reactions were conducted at room temperature using mixtures of 95-98% concentrated sulfuric acid and 27-33% fuming sulfuric acid with different acid ratios in order to get SPPs with degree of sulfonation (DS) in the desired range of 1.00-1.37. The presence of sulfonic acid groups in SPPs was confirmed by FT-IR analysis, and the DS and structures were characterized by NMR. The introduction of sulfonic groups into the polymer chains led to a decrease in the decomposition temperature. Membrane films were cast from SPPs solution in N,N-dimethylacetamide (DMAc). Water uptakes and swelling ratios of SPPs membrane films increased with DS and temperature. Proton conductivities of all SPPs increased with DS and temperature, reaching higher than 10(-2) S cm(-1) at around DS 1.0. (C) 2003 Elsevier B.V. All rights reserved.