In this work homogeneous sulfonation kinetic and both, rnicrostructural and electrical characterizations of hydrogenated styrene butadiene block copolymer (HSBS) were studied. The incorporation of sulfonic groups was checked by infrared spectroscopy (FTIR-ATR) and the sulfonation level was determined by both, elemental analysis (E.A.) and chemical titration. Thermal behavior was studied by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), in order to determine thermal transitions and stability, respectively. Microstructure was studied, by means of dynamic mechanical analysis (DMA). Atomic force microscopy (AFM) was studied for several samples and evidenced a morphology change when the Sulfonation level increases. Finally, electrical behavior was recorded by means of electrochemical impedance spectroscopy (EIS) and the four-probe technique (FPT). Results show that sulfonation of the benzene rings, in the styrene units, has effectively occurred and that the sulfonation level increases with reaction time. When incrementing the sulfonation level, a structural change in the copolymer was observed. Interconnected clusters allow proton conduction through the membrane. The ion conductivity obtained was about 10(-2) S/cm. (C) 2007 Elsevier B.V. All rights reserved.