An ultra-thin, flexible proton-conducting (PEI-PCGF)/phosphosilicate/(PEI-PCGF) (PPP) membrane with organic/inorganic multilayer structure is prepared. The total thickness of the membrane is ca. 500 nm. In the multilayer structure, the brittle inorganic phosphosilicate layer was supported and protected by organic PEI-PCGF layers, which provide sufficient mechanical strength and flexibility. The proton conductivity of the multilayer membrane is 10(-5) S cm(-1) and the area specific resistance is 4.5 Omega cm(2) at 70 degrees C, 90% relative humidity (RH). (C) 2011 Elsevier B.V. All rights reserved.