This work reports the preparation and characterization of hybrid membranes cast from dispersions of inorganic fillers in sulfonated polystyreneblock-poly(ethylene-ran-butylene)-block-polystyrene solutions. Silica gel, SBA-15 and sepiolite, all of them functionalized with phenylsulfonic acid groups, were used as fillers. For comparative purposes, the performance of composite membranes cast from dispersions of functionalized inorganic fillers in Nafion (R) solutions was investigated. Inspection of the texture of the membranes by using SEM techniques shows that the fillers are better dispersed in sulfonated polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene than in Nafion (R). The value of the water uptake for the membranes prepared from the former polyelectrolyte is in most cases at least three times that measured for hybrid Nafion (R) membranes. The conductivity of the membranes was measured at 80 degrees C by impedance spectroscopy obtaining values of 3.44, 6.90 and 3.54 S m(-1) for the hybrid membranes based on the triblock copolymer containing functionalized silica gel, SBA-15 and sepiolite fillers, respectively. These results compare very favourably with those obtained at 80 degrees C for Nafion (R) hybrid membranes containing silica gel, SBA-15 and sepiolite, all of them fuctionalized with phenylsulfonic acid groups, whose conductivities are, 2.84, 6.75 and 3.31 S m(-1), respectively. Resistance measurements carried out under controlled humidity conditions show that the conductivity of sulfonated triblock copolymer membranes containing functionalized SBA-15 filler undergoes a rather sharp increase when they are conditioned under an atmosphere of 75%, or larger, relative humidity. (C) 2007 Elsevier B.V. All rights reserved.