The overall mass transfer coefficient kLa, F in the flow characteristics was determined by the measurement of the diffusivity of ozone, density of aqueous solution, and viscosity. However, the measured values kLa, F in the range of 0.0096-0.0622 min-1 show large changes in hydraulic retention time, and the dissolved ozone concentration CL, F presented under 0.1 mg/l is lower than the dissolved ozone observed. The overall mass transfer coefficient kLa,M in the ozone decomposition was determined by measurement of the equilibrium dissolved ozone, overall decomposition rate constant, and overall Henry’s law constant. The measured values kLa, M are in the range of 0.0441-0.0749 min-1, and they present small changes depending on the hydraulic retention time. Furthermore, the measured dissolved ozone concentration CL,M presents a larger value than the CL, F. Then, the kLa,M is selected as an input overall mass transfer coefficient to predict the dissolved ozone requirement in the ozone contactor.