A new reflectance anisotropy (RA) spectrometer, in the 0.23–0.83‐&mgr;m range, is presented. The numerous similarities with phase‐modulated ellipsometry (PME) are emphasized. In particular, the RA spectrometer takes advantage of the high‐frequency modulation (50 kHz) provided by a photoelastic modulator. The use of optical fibers in both optical arms allows an increase of the compactness of the spectrometer. Four detectors can be used simultaneously providing the real‐time spectroscopic capability. The numerical data acquisition system of the detected signal is based on the use of a high precision analog‐digital converter and a fast Fourier transform processor. However, as compared to ellipsometry, RA can be only sensitive to the crystal surface. The adaptation of RA to a III‐V growth reactor by metalorganic chemical vapor deposition is described in detail. The high sensitivity of the RA spectrometer is emphasized. In particular real‐time variations of the RA signal ranging from 10−4to 10−3are reported. Then, the various RA techniques are compared. In particular, it is shown that this RA spectrometer allows the determination of both the real and the imaginary part of the signal. Finally the origin of the RA signal is discussed.