This work reports on the synthesis (via heterogeneous sulfonation) structural and electrical characterization of hybrid membranes based on block co-polymer ionomers (HSBR and EPDM) and silica gel prepared by sol-gel reaction. The structural characterization consisted of the analysis of their thermal and mechanodynamical transitions by DSC and DMA, respectively. The ion-exchange capacity of each polymer was determined by both titration and elemental analysis (EA). The introduction of sulfonic groups was verified by means of infrared spectroscopy (ATR). The electrical characterization was made using ac impedance at different hydration times, the conductivity being calculated from the corresponding impedance spectra. Finally, methanol crossover through the membranes was carried out, comparing the results obtained with Nation 117. The results indicate the existence of a complex microstructure formed by different phases corresponding to both ionic and non-ionic blocks of the co-polymer as well as the aggregates or clusters owed to the electrostatic interaction among ion pairs. Conductivity values are similar to Nation and they improve with hydration time for hybrid membranes due to the absorbent nature of the inorganic filler. Likewise, methanol crossover is lower than in Nation, probably due to the barrier effect exerted by the non-sulfonated blocks of the co-polymer. © 2005 Elsevier Ltd. All rights reserved.