Recent investigation has shown that ozone can be efficiently prepared by electrolysis of perchloric acid at low temperatures. Free of nitric oxide and other acid impurities, electrolytic ozone was directly obtained at concentrations above 20 weight‐per cent. Depending on cell design, the diluent gases are either oxygen or a mixture of hydrogen and oxygen. We have also found that a variable heretofore generally overlooked in current efficiency studies—the effect of reducing the absolute pressure over the electrolyte—can have a marked effect on current efficiency. Improved ozone yields are obtained at absolute pressures of 0.1 atmosphere. With cells of the type reported herein, eutectic mixtures of perchloric acid and water give excellent results at temperatures below −50°C. Sodium and magnesium perchlorates may be added to perchloric acid solution to give slightly lower melting points, with indication of some increase in current efficiency. Provided a partially frozen electrolyte is used, current efficiency does not vary appreciably over a wide current density range. Smooth platinum is a satisfactory anode material and silver‐free lead is suitable for the cathode. The consumption of platinum is known to be less—probably much less—than 10 milligrams per kilogram of ozone. The combined mechanical and chemical losses of perchloric acid are less than 1.7 grams per kilogram of ozone. For the direct preparation of relatively pure Ozone at high concentrations, the process described is more efficient than the silent electric discharge method.