Uniform, defect-free nanofilms (around 100 nm thick) of hafnium phosphate were prepared via layer-by-layer deposition of precursor solution of metal alkoxides and were shown to give practically useful proton conductivity at 300-400 degrees C as fuel cell electrolyte membrane. Annealing of the deposited precursor film at 500 degrees C gave a lower area specific resistance (R-AS) than that at 400 degrees C, and this effect was coincident with the formation of the pyrophosphate unit. The best (lowest) R-AS value of less than 0.1 Omega cm(2) for hafnium phosphate membrane (annealing at 500 degrees C, 291 nm thick, conductivity of 3 x 10(-4) S cm(-1) at 320 degrees C) was comparable to that of a nanofilm of an yttrium-doped zirconium phosphate. The RAS values of nanofilms of zirconium sulfate and zirconium borate were 2-3 orders higher than that of the corresponding zirconium phosphate. It is clear from these results that nanofilms of various solid acids are promising candidates for the electrolyte membrane of fuel cells operating at the intermediate temperatures of 300-400 degrees C. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3432600] All rights reserved.