An automatic ellipsometer based on the principle of measurement of intensities is described. Use is made of a rotating analyzer with a three‐channel strobing voltmeter to read the photomultiplier output at chosen analyzer angles. The outputs are recorded in digital form, and the time for one optical measurement is 150 msec. Sources of error and ways of eliminating noise from the signal are discussed. The system incorporates a cell specially engineered for electrochemical investigations, and its performance is illustrated with results from an experiment on the electrochemical formation of a thin oxide layer on platinum.