Fabrication of a nanoporous tungsten oxide film via anodizing of tungsten in an ammonium nitrate (NH4NO3)/ethylene glycol (EG)/water (H2O) mixture is reported for use as a visible light-driven photocatalyst. Anodizing of tungsten in a 1.0 M NH4NO3/EG solution containing less than 0.1 vol% H2O resulted in active dissolution of the tungsten substrate. As the H2O concentration increased to more than 25 vol%, a thin barrier oxide film was formed on the tungsten substrate. A thick porous tungsten oxide with numerous nanopores measuring several tens of nanometer in diameter was fabricated via anodizing at a moderate H2O concentration of 1.0 vol%. The porous oxide consisted of a double-layered structure with an outer porous layer and an inner dense layer, and the outer porous layer became thinner as the NH4NO3 concentration decreased. A uniform porous oxide film from the top surface to the bottom interface was fabricated via anodizing at 20 V in a 0.02 M NH4NO3/EG solution containing 1.0 vol% H2O at 313 K. The porous tungsten oxide exhibited visible light-driven photocatalytic activity for the photocatalytic decomposition of methylene blue. (C) 2017 Elsevier B.V. All rights reserved.