Numerical calculations were conducted to analyze the ozone absorption behavior and removal of odorous materials in the U-Tube(R) reactor, which is a bubble column fitted with a concentric down-flow diffuser, A novel calculation model was developed to simulate the reactor by assuming that it is composed of a plug flow section (inner tube) followed by a tanks-in-series section (outer bubble column), The bubble size distribution and gas holdup were measured in the downflow diffuser to obtain correlations for the gas holdup and the Sauter mean bubble diameter. It is found that the ozone absorption efficiency increases with decreasing gas/liquid ratio but is almost independent of the gas-phase ozone concentration. It is also found that the removal efficiency of odorous compounds (2-MIB) significantly increases with both increasing gas-phase ozone concentration and gas/liquid ratio. In order to achieve a high 2-MIB removal efficiency a ozone loading value (= gas-phase ozone cone. x gas/liquid ratio) greater than 0.003 kg/m(-3) is required. The numerical calculation well explains the experimental data on the ozone absorption efficiency and the 2-MIB removal efficiency of a pilot plant, showing usefulness of the simulation model proposed.