Abstract: A concentric-tube type ozone generator wherein the lower end portion of the interior of the tube contains a supply of oil which contacts the inner side of the inner tube wall above the inner electrode to prevent the flow of creep currents between the inner and outer electrodes. A second supply of oil can be confined in the upper end portion of the inner tube wall above the inner electrode. The inner electrode may consist of a metallic foil which is biased against the inner side of the inner tube wall by a helical spring. The outer electrode may constitute a helically convoluted metallic wire. Two or more tubes can be mounted above each other with the outlet at the lower end of the upper tube in communication with the inlet at the upper end of the lower tube.