For increased efficiency of high-temperature polymer electrolyte membrane fuel cells (HT-PEMFC), new types of membranes have to be developed. This approach has been realized by preparing hybrid membranes containing SO3H-functionalized mesoporous Si-MCM-41 as hydrophilic inorganic modifier in a polysiloxane matrix exhibiting sulfonic acid groups and basic heterocyclic groups like benzimidazole. The proton conductivity of sulfonated particles was modelled on the atomic scale in order to understand the influence of the density of sulfonic acid groups and of the presence of water molecules. The different hybrid membranes are characterized concerning their thermal stability, water uptake, and proton conductivity. Whereas the proton conductivity of well-established, but expensive and at >120 degrees C not long-time stable Nation membranes continuously decreases with increasing temperature, the polysiloxane membranes, which suffer from a low-proton conductivity at around 100 degrees C, recover at about 120 degrees C due to intrinsic proton transport. At 180 degrees C the pure polysiloxane shows a proton conductivity which is only one order of magnitude lower than that of Nation. Moreover, if the polysiloxane membrane contains additionally 10 wt.% of an SO3 H-modified Si-MCM-41, the proton conductivity of such hybrid membrane at temperatures > 180 degrees C and low relative humidity <10% is higher than that of Nafion membranes by a factor of 10. (C) 2007 Elsevier B.V. All rights reserved.