Thin, boron-doped diamond films formed on : a silicon substrate were evaluated during water electrolysis in acidic solution in order to determine their potential for industrial use; Though the electrode exhibited overvoltages in excess of 2 V in the industrial current range, ozone gas was produced at a : current efficiency of a few percent at ambient temperature. IL was confirmed that the consumption rate of the highly doped-sample was small and comparable with a platinum-plated anode, indicating that the diamond is dimensionally stable under extreme conditions. The failure mechanism in the test is discussed on the basis of scanning electron microscopy, X-ray diffraction and Raman analyses. The spalling of the film from the substrate, which was observed in the deteriorated sample after the electrolysis, is attributed to the residual stress that accumulated during the production process carried out under high temperature.