Tungsten oxide thin films were grown on glass substrates by RF sputtering at room temperature using a tungsten trioxide target for several values of the argon pressure (P(Ar)). The structural and morphological properties of these films were studied using X-ray diffraction and atomic force microscopy. The as-deposited films were amorphous irrespective of the argon pressure, and crystallized in a mixture of hexagonal and monoclinic phases after annealing at a temperature of 350 degrees C in air. Surface-roughness increased by an order of magnitude (from 1 nm to 20 nm) after thermal treatment. The argon pressure, however, had a strong influence on the optical properties of the films. Three different regions are clearly identified: deep blue films for P(Ar)<= 2.67 Pa with low transmittance values, light blue films for 2.67 Pa= 6 Pa with high transmittance values. We suggest that the observed changes in optical properties are due to an increasing number of oxygen vacancies as the growth argon pressure decreases. (C) 2009 Elsevier B.V. All rights reserved.