In this study, tungsten oxide nanoparticles were fabricated by pulsed laser ablation (PLA) of tungsten target using the first harmonic of a Nd:YAG laser (1064 nm) in deionized water. After ablation, a 0.2 g/lit PdCl2 solution was added to activate the solution against the hydrogen gas. Dynamic light scattering and X-ray photoelectron spectroscopy were used to measure the average size and the surface chemical composition of the synthesized nanoparticles, respectively. The aim is to investigate the influence of hydrogen exposure time on colouration process of colloidal nanoparticles. According to optical measurements, hydrogen bubbling into the produced colloidal Pd-WO3 led to formation of several absorption peaks at similar to 1.26, similar to 1.6 and similar to 1.97 eV. We observed the appearance and growth of a peak at 1.6 eV at the initial stages of hydrogen exposure. However, two other peaks became dominant at long exposure times. The coloration process is reversible in the presence of oxygen gas. (C) 2012 Elsevier B.V. All rights reserved.