Thin films of amorphous and crystalline tungsten trioxide were deposited onto glass covered with indium tin oxide, by reactive dc magnetron sputtering. The samples were intercalated with Li ions in 1M LiClO4 in propylene carbonate. The infrared reflectance was measured ex-situ at wavelengths between 2 and 50 micrometers for LixWO3 of different thickness, d, and at different Li/W ratios, x. The absorption bands (i.e. the dips in the reflectance spectra) of a thick amorphous film (d equals 1450 nm) get stronger up to the threshold value x equals 0.17. Above this value, the amplitude of the oscillations in the reflectance spectra decreases, which resembles the behavior of a thick crystalline film (d equals 1270 nm). The latter seems to have a lower threshold value, around x equals 0.08 and the spectra at large x are flatter than the ones for the amorphous film.