Novel proton-conducting ZrO2-WO3-SiO2 nanofilms of various compositions and thicknesses (similar to 50 to similar to 300 nm) have been prepared by anodizing of magnetron-sputtered Zr-W-Si alloys in 0.1 mol dm(-3) phosphoric acid electrolyte at 20 degrees C. All the anodic oxide nanofilms examined reveal efficient proton conductivity after post-annealing at 250 degrees C. Further increase in the post-annealing temperature results in the conductivity degradation for the anodic oxide nanofilms on the alloy containing only 5 at% silicon, while the high conductivity is maintained even after post-annealing at 300 degrees C for those containing 15 at% or more silicon. The proton conductivity is dependent upon tungsten content; the conductivity of 5 x 10(-6) S cm(-1) for the similar to 100 nm-thick films on the Zr31W55Si14 at 100 degrees C is approximately 10 times that on the Zr48W37Si15. The anodic oxide nanofilms consist of two layers, comprising a thin outer ZrO2 layer and an inner ZrO2-WO3-SiO2 layer. Both layers show thickness-dependent conductivity and the proton conductivity of the two-layer anodic films is enhanced one order of magnitude by reducing the film thickness from similar to 300 nm to similar to 100 mu. Different mechanisms are proposed for the thickness dependence of the conductivity of the outer and inner layers. (C) 2013 The Electrochemical Society. All rights reserved.