A new calibration method for FTIR phase-modulated ellipsometry is presented. This procedure is based on the multiple-harmonic modulation model and takes into account the effect of the multiple reflections of the beam between the modulator faces. With this procedure all the Bessel function background in the wavenumber range from 950 to 3500 cm(-1) is removed. When this is achieved, the ellipsometric angles Psi and Delta can be determined accurately using any of the standard measurement configurations. The measurement of the thickness of an amorphous silicon dioxide thin film, thermally grown on a crystalline silicon substrate, is used as a quality test of the new calibration procedure. FTIR phase-modulated ellipsometry and UV-vis phase-modulated ellipsometry results agree within 0.1%, indicating that the FTIR measurements performed with the new optimized calibration method are accurate.