The ozone production rate of electrolytic ozone generators with proton conducting membranes and PbO2 anodes has been determined for demineralized water. At ambient temperature and a current density of 1 A/cm² it is 18%. Experience shows that the presence of dissolved salts or organic impurities reduce the current efficiency. A realistic value is 8%. One reason is the reduced catalytic reaction at the electrode. Another reason is that ozone quickly reacts with organic substances thus distorting the current efficiency result. For an exact determination of the performance the current efficiency for a given water quality needs to be determined experimentally by measuring the concentrations of ozone in water and gas.