The preparation of sulfonated polybenzimidazole by sulfuric acid treatment on pre-formed polybenzimidazole membranes was investigated. This polymer, which is originally a thermal resistant and insulating material is transformed by the chemical treatment into a proton conductor. The modified material is stable at temperatures close to 400 degreesC. The proton conductivity of the sulfonated membrane at 160 degreesC and 100% r.h. is 7.5 x 10(-5) S/cm. The low conductivity is explained by protonation of nitrogen in the imidazolium ring, as evidenced by FT-IR analysis, which lowers the proton mobility. Hydrogen bridge bonds are formed between nitrogen in the imidazole and oxygen of sulfonic group creating a more regular structure in the material which becomes insoluble in polar solvent such as dimethyl sulfoxide. These modifications of the material have been observed in the: X-ray diffraction patterns, which evidence an incipient crystalline structure of the sulfonated polybenzimidazole.