In this study, WO3 nanostructured films were prepared by sol-gel spin coating process on glass substrates and drops of PdCl2 solution were used as an aqueous hydrogen catalyst for gasochromic investigations. As deposited WO3 films were annealed at different temperatures of 100, 200, 300 and 400 degrees C in air. The effect of annealing temperature on crystalline structure and surface morphology of WO3 films was studied by X-ray Diffraction (XRD), Field Emission Scanning Electron Microscope (FE-SEM) and/or Atomic Force Microscope (AFM). Fourier Transform Infrared (FTIR) spectroscopy revealed the variation of chemical bonds with annealing temperature. Because of the aqueous nature of hydrogen catalyst, hydrophilicity of samples was measured by means of contact angle and it was found to increase with annealing temperature. We found a correlation between annealing temperature, hydrophilicity, palladium growth and coloring of tungsten oxide films when an aqueous hydrogen catalyst is used. The effect of catalyst concentration on colored state optical density was also studied for films annealed at 400 degrees C. (C) 2012 Elsevier B.V. All rights reserved.