The water uptake, transport and mechanical properties of organic-inorganic hybrid materials based on sulfonated and silylated, cross-linked polyetheretherketone (called SOSi-PEEK/N, where N is the molar percentage of silylated monomeric units) for application in fuel cells are reported. The mechanical properties are superior to those of S-PEEK, due to the presence of silicon. The water and proton diffusion coefficients are reported as function of temperature and relative humidity. The proton conductivity of the membranes increases nearly four orders of magnitude when relative humidity varies from 30% to 100% corresponding to an increase of water/equivalent of ion-exchange groups,lambda, from 1 to 15. The activation energy for proton conduction is slightly higher than in Nation. Proton diffusion coefficients obtained from the Nernst-Einstein equation are compared with water chemical diffusion coefficients determined from water sorption kinetics. The equivalence of both quantities for; >= 4 indicates that proton conductivity takes place via Grotthuss-type mechanism. (c) 2007 Elsevier B.V. All rights reserved.