The effects of water flow rate and current density on the ozone concentration in anode water were investigated using a new electrochemical ozone production (EOP) system. The absorption coefficient of O-3 at 290 nm in water was found to be 720 L mol(-1) cm(-1), about twice as large as that in the gas phase, 374 L mol(-1) cm(-1). The O-3 concentration increased with increasing current density and decreasing anode water flow rate. The highest O-3 concentration of 70 wt ppm was observed when the water flow rate was 100 mL/min, the current density was 2.13 A/cm(2), and the cell pressure was 0.2 MPa. This concentration is the highest reported for a once-through EOP system. As the anode water flow rate decreased, the current efficiency of O-3 production also decreased owing to the formation of O-3 and O-2 gases, which reduce the catalytically active area of the Pt anode. The high current efficiency of 29.1% at a water flow rate of 820 mL/min decreased to as low as 6.7% at 92 mL/min. The results of pressure dependence measurements of the current efficiency of O-3 production and the O-3 concentration in anode water suggest that high-pressure operation has a slightly positive effect on them. (C) 2014 Elsevier Ltd. All rights reserved.