The mechanism of photoelectrocatalytic dehydrogenation of 2-propanol on a ZnO photoelectrode is re-examined. The effect of gaseous atmosphere is studied. The change of pH due to the reaction is followed under different experimental conditions. The larger part of the dissolution of ZnO occurs in the dark itself due to the solubility of ZnO. The photodissolution is comparatively small. 2-Propanol inhibits the photocorrosion slightly. H2O2 is detected in the electrolyte irrespective of the presence of 2-propanol. An electron transfer process followed by a free radical mechanism for the formation of H2O2 is proposed and discussed. © 1983.