We present a detailed study of the morphology and composition of tungsten oxide (WO3) thin films, grown by radio frequency magnetron reactive sputtering at substrate temperatures varied from room temperature (RT) to 500 °C, using infrared (IR) absorption, Raman spectroscopy, and x-ray photoelectron spectroscopy (XPS). This work includes valuable new far-IR results about structural changes in microcrystalline WO3. Both IR absorption and Raman techniques reveal an amorphous sample grown at RT and initial crystallization into monoclinic structures for samples grown at temperatures between 100 and 300 °C. The Raman spectra of the samples grown at high temperatures indicate, apart from the monoclinic structure, a strain effect, with a distribution revealed by confocal Raman mapping. XPS indicates that the film surface maintains the stoichiometry WOx, with a value of x slightly greater than 3 at RT due to oxygen contamination, which decreases with increasing temperature.