This paper reports the gasochromic effect in colloidal nanoparticles of tungsten oxide dihydrate synthesized via a simple electrochemical anodizing of tungsten in an electrolyte composed of 0.02 M HCl. The anodizing caused the electrolyte to convert to a light yellowish colloidal solution consisting of tungsten oxide dihydrate. The ultimate gasochromic colloidal solutions were obtained just by addition of different volumes of 0.2 g/l PdCl2 solution as a source of hydrogen catalyst into the primary colloidal tungsten dihydrate solution. Different tools involving, X-ray diffraction (XRD), high-resolution transmission electron microscope (HRTEM), X-ray photoelectron spectroscopy (XPS), and furrier transformed infrared (FTIR) spectroscopy characterized the colloidal nanoparticles. XRD measurements revealed that the as-prepared nanoparticles are monoclinic WO3 center dot 2(H2O) and were converted to monoclinic WO3 by annealing at temperatures above 300 degrees C. XPS showed that the hydroxyl groups and W5+ states are dominant in as-prepared sample and lower after annealing or loading PdCl2. It was observed that the Pd-WO3 center dot 2(H2O) solutions which were colorless initially, turned into blue color after dilute hydrogen insertion and then into the colorless state by spontaneously bleaching in ambient air. Moreover, no coloring was observed for colloidal solution composed of monoclinic WO3. The optical absorption spectra of colloidal samples in colored states were composed of three certain distinct absorption peaks located at 1.3, 1.6 and 1.9 eV. The intensities of first and third peaks were comparable and dominant at deep blue states but, upon bleaching, the second one gradually overcomes them. We attributed this dynamical behavior to the possible surface and bulk phenomena. Finally, the obtained optical absorption results were compared with the small polaron hopping model. (C) 2014 Elsevier B.V. All rights reserved.