Homogeneous, transparent and crack-free P2O5-ZrO2 and P2O5-ZrO2-SiO2 membranes have been synthesized by the sol-gel process. A first step has been oriented to the optimization of the synthesis and characterization of different compositions by TGA, FE-SEM, FTIR and EIS to choose the best inorganic composition in terms of chemical and mechanical stability, and proton conductivity. The addition Of SiO2 improves the mechanical and chemical stability. On the other hand, compositions with higher content in P2O5 have demonstrated lower mechanical and chemical stability against water, but higher proton conductivity. The water retention and high porosity of inorganic membranes leads to high proton conductivity, 10(-2) S/cm, at 140 degrees C and 100% relative humidity. The second step has been focused in the study of doped inorganic membranes of molar composition 99.65(40P(2)O(5)-20ZrO(2)-4OSiO(2))-0.35PWA. The high homogeneity, transparency and SEM-EDX analysis of these membranes indicates no phase separation suggesting that PWA is well dispersed in the inorganic structure. The incorporation of PWA in sot-gel oxides provides an increase of the proton conductivity at low relative humidity due to the adequate distribution of PWA in the inorganic network. Conductivity increases in two orders of magnitude at low humidity (10(-4) S/cm at 50 degrees C and 62% RH) compared with undoped sol-gel oxide membranes. (c) 2007 Elsevier B.V. All rights reserved.