A circulating water electrolysis systemwas developed to produce highly concentrated O-3-dissolved water (ozone water). It enabled the production of 160 and 112mg/L ozone water in batch and continuouswithdrawal operation, respectively. The ratios of the actual gas-phase O-3 concentrations to those calculated from the dissolved O-3 concentration using Henry's law at 1 atm were about 0.5, suggesting the supersaturation of ozone water. Once the O-3 gas dissolved in the electrolysis cell operated at 0.15 MPa/G, it should have taken time for the ozone water to reach the gas-liquid equilibrium. The electric conductivity of the ozone water increased to 1.2 mS/cm and the formation of ClO2 was observed at the end of the run. The electrophoretic migration of Cl-ions from the catholyte to the ozone water is considered to be the cause of this impurity. The time dependence of the dissolved O-3 concentration and the maximum dissolved O-3 concentration obtained in the experiment were well reproduced by a circulating water electrolysis simulator. (c) 2018 Elsevier Ltd. All rights reserved.