AbstractThe kinetics of the competition between the anodic decomposition of the dark p‐GaAs electrode and the anodic oxidation of TMPD have been studied in aqueous and in mixed aqueous‐methanolic medium by rotating ring‐disk voltammetry. The results were found to be qualitatively and quantitatively in agreement with those, previously obtained from an analogous study on the n‐GaAs photoanode, and confirm the conclusion that the mechanism depends on the medium. The latter point is further supported by the relationships between anodic current density, potential and TMPD concentration, observed at p‐GaAs. The accepted competition mechanisms allow to propose structures for the surface decomposition intermediates of GaAs involved. The value of the rate constant for the first decomposition step of GaAs is determined from current density‐potential and impedance measurements on p‐type electrodes and