The evolution of different W oxidation states and the oxide reduction mechanism of polycrystalline WO 3 thin films, induced by low-energy H 2 + bombardment at room temperature, was investigated in situ by X-ray photoelectron spectroscopy around W 4f and O 1s core levels and the valence band. A hydrogen tungsten bronze is formed at the beginning of the reduction process, as is evident from the development of the W 5+ oxidation state and the creation of the O−H bonds. With the higher H implantation dose, reduction proceeds with the creation of H 2 O gas molecules, whose evolution with the bombardment time correlates with the cumulative concentration fractions of W 4+ , W 2+ , and W 0 oxidation states. The generation of H 2 O molecules removes O atoms from the WO 3 matrix, inducing the reduction of WO 3 to lower oxides and, subsequently, metallic W, which is the dominant phase on the surface after 180 min of bombardment.