Tungsten trioxide (WO2.9) nanoparticles were prepared, to our knowledge for the first time, by pulsed laser ablation in distilled water. The experiments were performed irradiating a tungsten target with a second harmonic (532 nm) Nd:YAG laser beam varying the operative fluence between 1 and 7J cm(-2) and the ablation time up to 120 min. As evidenced by means of transmission electron microscopy (TEM), at all investigated fluences, small nanostructures of 2-6 nm were accompanied by larger particles with a diameter of about 10-20 nm and aggregates of about 80-100 nm. A plasma shielding effect was evidenced upon increasing the laser fluence, while if the ablation time is increased the amount of particles increases as well, supporting the scalability of the production technique. The deposited nanoparticles stoichiometry has been verified by X-ray photoelectron spectroscopy (XPS), while the optical bandgap values were determined by UV-vis optical absorption measurements. (C) 2011 Elsevier B.V. All rights reserved.