Résumé : The photosensitivity increase of CdS films as a function of the illumination time, is correlated to chemical changes of the electrode surface, analysed by XPS. It is shown that the formation of Cd(OH)2 or CdO is responsible for the photosensitivity change of the illuminated electrode in contact with H2O/NaOH alone. When thiocresol or p‐mercaptobenzoic acid is added as reductant, a layer of organic compounds is formed on the electrode during the photoelectrochemical process. However, the XPS and photocurrent spectra indicate that this layer does not prevent the occurrence of Cd(OH)2 or CdO responsible for the photosensitivity changes of the CdS film. Copyright © 1985 Wiley Heyden Ltd.