A novel ellipsometer was set up that uses phase-sensitive detection to characterize the state of polarization of the optical radiation. Before reflecting from the sample surface, the incident polarization of time optical beam is altered and modulated by a fixed retarder followed by a second retarder that rotates at frequency nu = omega/2 pi. After reflection, the beam passes through a linear polarizer and the phases phi(2) and phi(4), of the respective 2nd and 4th harmonics of the output signal are measured. The ellipsometric angles Delta and Psi are fully determined from the values of phi(2) and phi(4) over the entire domain (0 degrees < Delta less than or equal to 360 degrees; 0 degrees < Psi less than or equal to 90 degrees). In the reported experiment the retarder was rotating st 20 Hz, which corresponds to a time resolution of 25 ms. The calibration and testing of the phase-sensitive ellipsometer an reported together with its application as an optical diagnostic during the coloration process of electrochromic WO3 films.