Nowadays, gasochromic Pd/WO3 coatings as optically switchable materials have become more applicable for hydrogen sensors and smart windows. In this study, WO3 films were prepared by Pulsed Laser Deposition (PLD) and spin-coating sol-gel techniques. For deposition of Pd, first a layer of PdCl2 was obtained via a simple drop-drying process by dropping PdCl2 solution onto WO3 substrates and drying them at room temperature. Then Pd nanoparticles were synthesized via hydrogen gas exposure that causes reduction of the PdCl2 layer. According to Scanning Electron Microscope (SEM) observations before hydrogen reduction, many individual nanoparticles or fractal-like constructions of palladium were formed in the PdCl2 layer in which the fractal branches were distorted after hydrogen treatment. Surface chemistry of the observed Pd nanoparticles was studied using X-ray Photoelectron Spectroscopy (XPS) at different stages of the reduction process. The results showed that after hydrogen treatment, the chlorine atoms were desorbed from the PdCl2 layer and a metallic Pd layer remained on the surface of WO3. Gasochromic properties in the presence of H-2 or O-2 gases for different PdCl2 amounts revealed that the rate and saturated level of coloring depends on the PdCl2 amounts as well as on the preparation method of the WO3 substrates due to different porosities. Crown Copyright (C) 2011 Published by Elsevier B.V. All rights reserved.