The effects brought about by the time of thermal treatment as well as the water content in the gas phase during the thermal spreading of WO3 on zirconia were investigated. Diffuse reflectance UV-vis spectroscopy evidenced the thermal spreading phenomenon and revealed the formation of polymeric tungsten dispersed species. Neither the thermal treatment time nor the water content showed to influence the nature of the dispersed species, which reveal to present thermodynamically preferential molecular structures. Infrared spectroscopy analysis of adsorbed pyridine evidenced that the polytungstate species lead to the generation of Bronsted acid sites. Lewis acid sites stronger than those naturally present on zirconia could also be detected in addition to weaker Lewis sites, which were associated to the WO3 still present in the catalysts as showed by X-ray diffraction. (c) 2008 Elsevier B.V. All rights reserved.