The structure, surface acidity, and reducibility of WO3/SiO2 systems, Obtained by impregnation of the silica support with aqueous paratungstate solutions, are described. The effect of tungsten loading and calcination temperature on the nature and physicochemical properties of tungsten-containing phases has been studied by FT-IR, Raman and W-L-3 XAS spectroscopies, X-ray diffraction, temperature-programmed reduction, and FT-IR monitoring of pyridine adsorption for surface acidity. Results obtained show that, due to a reaction between the tungstate and the support, calcination up to 723 K leads to formation of a Si-O-W crystalline species where tungsten is forming Keggin-type units similar to those found in dodecatungstosilicates. As the calcination temperature is raised, these species decompose leading to crystalline WO3, due to the low dispersing ability of silica. Thermal satiability of this Si-O-W species is higher than that of commercial dodecatugstosilicic acid, although it slightly decreases as the tungsten content decreases. Si-O-W species possess large surface acidity, with both Lewis and Bronsted acid sites, and they are more easily reduced than crystalline WO3.