Far-infrared transmittance and reflectance of PbTiO3, PbZr0.53Ti0.47O3 and PbZr0.75Ti0.25O3 thin films deposited by a sol-gel technique on sapphire substrates were measured between room temperature and 650 degrees C. The spectra were fitted with a classical oscillator model to calculate the dielectric response. Polar-mode parameters are more accurate than data on bulk ceramic materials and are in good agreement with them. An analysis of the soft-made behaviour and comparison with the low-frequency permittivity data clearly indicate the existence of additional relaxation that is always present in the several cm(-1) range, even at room temperature.