Successful utilization of direct methanol fuel cell (DMFC) technology is retarded by the lack of sufficient membrane materials. An improved performance of organic membranes containing inorganic fillers in hydrogen-fuel cells is known, however, microstructure-function relation of these materials is not yet well understood. In order to gain basic knowledge, the structural and functional properties of different Nation 117 membranes filled with zirconium phosphate were studied by numerous techniques like, e.g. cryo scanning electron microscopy to estimate the water to acid group ratio as a function of zirconium phosphate concentration, dynamic mechanical analysis, and X-ray diffraction (XRD) to localize the inorganic inclusion in the polymer and, finally, swelling kinetics in water and methanol to understand the effective solvent diffusion in hybrid membranes. The results show the water content of the membranes to be increased by the filler material, even at high concentrations of zirconium phosphate of five phosphoric acid groups per sulfonic acid group. The strong influence of the filler concentration on the mechanical loss and particularly on the glass transition as well as on crystallinity of the polymer back bone indicates major structural chances within the perfluorinated polymer due to the inorganic inclusions. From the solvent diffusion it could be shown, that zirconium phosphate selectively influences methanol diffusion and can act both as a stabilizer for the polymeric matrix and as a diffusion barrier for the methanol transport. (C) 2004 Elsevier B.V. All rights reserved.