Thin films of tungsten oxide deposited by hot filament metal oxide deposition (HFMOD) were thermally annealed up to 800 degreesC and investigated by means of XRD, Raman spectroscopy, and infrared reflection-absorption spectroscopy (IRRAS). As clearly shown by the XRD and Raman spectroscopy data, the deposited films were amorphous and crystallized by thermal annealing. The monoclinic WO3 phase was formed in all annealed samples. The IRRAS spectra were obtained using the IR beam with p-polarization and an off-normal incidence angle. In this condition, absorptions due to the longitudinal optical (LO) modes (Berreman effect) can be observed in the spectra. Absorptions due to LO modes are not detected by the standard infrared absorption spectroscopy, in which an unpolarized IR beam is used at normal incidence, and thus are not frequently reported in the literature. To analyze the experimental IRRAS spectra, the LO and TO functions were calculated from the transmission spectra of the as-deposited sample, using the Kramers-Kronig transformation and spectral simulation was carried out using the optical constants of both amorphous and crystalline WO3. For the as-deposited sample, the LO function of the films exhibited a very prominent band at around 950 cm(-1) which was also observed in the IRRAS spectra for all samples. For the annealed samples, this band shifted to higher wavenumbers and narrowed and a series of low-intensity bands appeared around 950 cm(-1), since crystalline structure changes were induced by thermal treatment. The results signal the applicability of the Berreman effect to the phase characterization of metal-supported WO3 films.