The optoelectronic properties of WO3 thin films were investigated after hydrogen treatment. Thin films were deposited on heated substrates by the thermal evaporation and were subsequently annealed at various temperatures in hydrogen for 2 h. Structural studies were performed using X-ray diffraction and atomic force microscopy. As-deposited films were amorphous and became crystalline by thermal annealing. The chemical properties were characterized by X-ray photoelectron spectroscopy, and revealed reduction in stoichiometry and the presence of oxygen vacancies in the films as a result of hydrogen annealing. Spectrophotometric measurements showed that the transmittance of the films was decreased consistently with hydrogen annealing. Significant changes in the optical constants and indirect band gap were noticed. Hall measurements showed the significantly reduced electrical resistivity due to the presence of oxygen vacancies. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.