With boron-doped diamond electrodes, an apparent cathodic peak was observed at 0 V in cyclic voltammograms obtained in 0.1 M HClO4 solution containing ozone. This peak was attributed to the reduction of dissolved ozone. A calibration curve obtained from the cathodic peak current, which has a slope of 6.7 mu A cm(-2) (mg L-1)(-1), can be applied to determining the dissolved ozone concentration. This result will lead to the development of an electrochemical ozone sensor employing a diamond electrode.