A new Fourier transform infrared phase‐modulated ellipsometer is presented. It combines the high frequency provided by a photoelastic modulator (37 kHz) with the low frequency of the Fourier transform infrared spectroscopy (<1 kHz), by means of a numerical data acquisition system. A full spectrum recording (from 900 to 4000 cm−1) can be achieved in 2 s. Thus, it allows its adaptation for kineticinsitustudies. The optical setup and the data reduction procedure are presented. In particular, a self‐consistent spectral calibration procedure is described in detail. The precision in &PSgr; and &Dgr; increases from 0.3° to 0.02° when increasing the integration time from 2 to 760 s. The examples shown in this article illustrate the high sensitivity to identify and analyze the absorption vibration variations of ultrathin films (a few angstroms thick).