Tungsten oxide thin films due to their gasochromic properties are widely considered to produce optical hydrogen gas sensors. In the gasochromic process, the color of tungsten oxide thin ?lms converts from a traarent to a dark blue state when atomic hydrogen incorporates into their lattice. This process is often reversible, i.e. whenever the colored ?lm is exposed to O 2 gas, the layer becomes bleached. Previous studies shown that, for a gasochromic coloration of WO 3 thin ?lms, it is necessary to deposit a very thin layer of metallic Pd or Pt as hydrogen catalyst over it. Pd is a well-known catalyst for hydrogen. In gasochromic devise, the characteristics of catalyst layer can determine the dissociation rate of absorbed gases and hence the gasochromic responses. There are different ways to deposit a thin Pd ?lm such as sputtering, e-beam evaporation, Electroless, and hydrogen reduction. Among them, hydrogen reduction is known as a simple, inexpensive, and relatively successful method for Pd layer deposition. In this method, a drops of PdCl 2 solution is put on the WO 3 surface until dry to get a PdCl 2 accumulated layer. By exposing hydrogen gas to dried-PdCl 2 /WO 3 sample, a layer involving distributed Pd nanoparticles grow on the tungsten oxide surfaces, through a reduction reaction. As a new approach, the droplet of PdCl 2 without drying stage can also be used in gasochromic coloring of WO 3 ?lms as an aqueous hydrogen catalyst. In this study, this idea was examined and an interesting gasochromic coloring effect was observed. In the following, a correlation between deposition method and preparation condition of tungsten oxide ?lms, palladium top- layer growth and gasochromic response of their using an aqueous hydrogen catalyst, was investigated. For this purpose, tungsten oxide thin films were fabricated by sol-gel and pulsed laser deposition methods. In sol-gel method, the effect of annealing temperature and in pulsed laser deposition method, the effect of substrate temperature on structural, morphological, chemical and optical properties of tungsten oxide thin films was studied by X-ray Diffraction (XRD), Atomic Force Microscope (AFM), Field Emission Scanning Electron Microscope (FE-SEM), Fourier Transform Infrared (FTIR) spectroscopy, and UV-VI spectrophotometer. Because of the aqueous nature of hydrogen catalyst, hydrophilicity of samples was measured by means of contact angle. These studies show the dependence of the properties of tungsten oxide thin films on deposition method and preparing condition of them. Gasochromic investigation of the samples using an aqueous hydrogen catalyst and also morphological study of obtained palladium layer showed the hydrophilicity of tungsten oxide layers has the effective role in palladium top-layer growth and consequently gasochromic response of them. Using aqueous hydrogen catalyst, it also was observed that the pulsed laser deposited tungsten oxide layers in comparison with sol-gel derived films have a better gasochromic response. This observation arises from the higher porosity and better hydrophilicity of the laser ablated tungsten oxide layer and more uniform distribution of palladium catalyst over the surface of them. Keywords: Tungsten oxide, Sol-gel, Pulsed laser deposition, PdCl 2 , Aqueous catalyst, Hydrophilicity, Gasochromic