Post-sulfonation thermal treatment (psTT) has been suggested in U.S. patent 4,634,530 for attaching a sulfonate group to the aromatic ring of polybenzimidazoles (PBI), which were shown to improve separatory capabilities of PBI membranes (PBIm). In this article, the influence of a psTT on conducting and thermal properties of sulfonated PBI (sPBI) membrane and the stability of these properties after a severe water-washing process were investigated. The nature of the interaction between sulfuric acid and PBI was also studied. Results have shown that most of the sulfuric acid molecules are ionically bonded to the imide ring and that the thermal-treated sPBI membranes retain more acid molecules after the washing process than the nonheating ones. Thermal stability of PBIm decreases with sulfonation, but it is high enough for their application in medium-temperature fuel cells. However, the psTT does not increase significantly the proton conductivity of sPBI membranes, although it improves the stability of conducting properties after the washing process, when the amount of sPBI units is less than 25%.