A series of sulfonated PPESK (SPPESKs) were synthesized through a heterogeneous sulfonation process with fuming sulfuric acid as sulfonating agent in a chloroform solvent. Membranes prepared from SPPESKs were investigated and proved to be candidates of proton exchange membrane in fuel cell operating at high temperature and low humidity. The heterogeneous sulfonation reaction is verified to first occur on the interface of the acid phase and the chloroform phase, then went on in the acid phase. SPPESKs with sulfonation degree (DS) up to 2.0 are obtained through a new reprecipitation method. Effects of reaction temperature, reaction time, acid/polymer ratio, and chloroform/polymer ratio on the sulfonation reaction are reported in details. An increase in sulfonation degree results in the increase of hydrophilicity, bringing about a substantial gain in proton conductivity. SPPESK membranes exhibit high water uptake of about 105.4% with DS of 1.01, almost two times higher than that of Nafion(c) with similar dimensional variation. Conductivity values at 35 degrees C, 60% R.H. ranging from 10(-3) to 10(-2) S/cm were measured, which are comparable to or higher than that of Nafion (R) 112 (1.635 x 10(-2) S/cm) under the same test condition. Thermogravimetric analysis shows that SPPESK membranes are stable up to 290 degrees C in N-2. (c) 2007 Wiley Periodicals, Inc.