In situ real time ellipsometry has been applied to control and monitor GaAs exposure to H-2, O-2 and N-2 plasmas for the processes of native oxide removal, oxidation and nitridation, respectively. Single wavelength ellipsometry is suitable for detecting the cleaning end-point and for defining the optimal cleaning conditions, whereas both single wavelength and spectroscopic ellipsometry measurements are needed to elucidate the chemistry and kinetics of the GaAs plasma oxidation and nitridation. In particular, the anisotropic UV photoenhancement of the GaAs oxidation rate in the initial stage (< 300 Angstrom) and the different chemistry and morphology of GaN layers obtained by GaAs nitridation with N-2, N-2-H-2 and N-2-NH3 plasmas are highlighted.