Ozone decolorization and mineralization of azo dye Orange II. were examined. In order to elucidate the dynamics of the oxidation process by ozone, the rate-based model consisting of the rates of chemical reaction and gas-liquid mass transfer was developed. The overall enhancement factor was introduced to correct the underestimation of mass transfer driving force for fast reactions. In modeling, nonideal mixing characteristics in the gas and liquid phases were described using a tanks-in-series model with backflow. The parameters in the proposed mixing model were determined on the basis of a multiple circulation cell model and a velocity profile of recirculatory flow in a bubble column. Experiments for decolorization and mineralization of Orange II were performed to verify the capability of the proposed model in a semibatch bubble column. The decolorization completed rather quickly. However, the mineralization characterized by the total organic carbon (TOC) was incomplete and its rate was rather slow. It was found that the decolorization rate decreased with the increase in the initial Orange II concentration and increased with increasing ozone gas injection rate, ozone dosage, and solution pH. The proposed model could reasonably describe the present experimental results for the dynamic changes and the steady state of decolorization, mineralization, dissolved ozone concentration, and ozone concentration in the outlet gas. The proposed model might effectively handle the prediction of ozone oxidation process in a semibatch bubble column.