Ceramic electrolytes operating in the temperature range of 200 to 500 degrees C under dry atmosphere are a key material for the next-generation fuel cell and related applications. We discovered that nanometer-thick films of amorphous hafnium silicate (HfnSi1-nOx) exhibited efficient ionic conduction at 100-400 degrees C in dry air. When the fraction of hafnium doping was around 0.1, the nanofilm showed a low area-specific-resistance (<0.15 Omega cm(2)) at around 350 degrees C that was small enough for the practical fuel cell application. The sub-100 nm-thick membranes of Hf0.13Si0.87Ox, could be fabricated on porous Pt/alumina substrate to provide gas concentration cells. The electromotive force observed with H-2 concentration cell indicated that the ceramic nanomembrane acted as predominant proton conductor without permeation of H-2 gas. In addition, the Hf0.13Si0.87Ox and Zr0.11Si0.89Ox membranes responded to the change of O-2 pressures in O-2 concentration cells, while the Al0.16Si0.84Ox and Ce0.06Si0.94Ox membranes did not produce the electrical voltage by gradient of O-2 pressure. We conclude that Hf0.13Si0.87Ox nanomembrane is promising as electrolyte material for fuel cells and related ionics devices. (C) 2009 Published by Elsevier B.V.