The mechanism of the two‐step reduction of the methylviologen cation radical (MV2+) was studied at CdS, GaAs and InP single crystal electrodes. In aqueous solution the second step (MV+• → MV0) gives rise to layer formation which is not observed in mixed ethanol/water solutions. This layer has interesting electrical properties. The reoxidation of MV0 occurs by electron injection into the conduction band of all three semiconductors. The reoxidation of MV+• at InP is also a conduction band reaction and electroluminescence is observed with the p‐type semiconductor. Electron injection from MV+• into the conduction band of GaAs is not observed while the kinetics of the reaction at CdS depend critically on the state of the electrode surface and on the nature of the electrolyte solution.

Spectrally resolved electroluminescence (EL) has been studied during anodic dissolution of p-lnP, p-GaAs, and p-GaP electrodes in indifferent aqueous electrolyte solutions. The most efficient EL was found for p-lnP whereas p-GaP showed no emission. The EL is attributed to radiative band-band recombination of holes with electrons excited from dissolution intermediates.