Remote radiofrequency H2 and O2 plasma processing of InP and GaAs surfaces was investigated by in situ real time spectroscopic ellipsometry. Hydrogen plasmas were used for the native oxide removal and the defect passivation of III-V surfaces. The effect of hydrogen exposure time and of crystallographic orientation (GaAs (100), (110), (111)) on the chemistry and kinetics of oxygen removal and of phosphorus/arsenic depletion was investigated. Oxygen plasma anodization was used to grow oxide films on GaAs (100), (110) and (111) substrates. The effect of bias voltage and UV-light irradiation on the chemistry and kinetics of oxidation process and on the oxide properties was studied. The composition and morphology of the InP and GaAs surfaces resulting from these plasma treatments was described.