The influence of pH value, UV intensity, and anion species on the decomposition of ozone in aqueous solution was studied for the batch and semibatch systems. The results indicate that the decomposition efficiency of ozone increased with increasing pH value and UV intensity. The two-film theory was adopted to describe the different self-decomposition behaviors of ozone for the batch and semibatch systems. The extent of the chain reaction between ozone molecule and hydroxyl radical in the semibatch systems was found to be much higher than that in the batch systems on the basis of the mass balance calculation. The decomposition of ozone for the semibatch systems was postulated to occur mainly in the film region by the free radical chain reaction. With ultraviolet irradiation, the decomposition rate of dissolved ozone was found to be not significantly affected by solution pH. The overall decomposition rate equation of ozone with UV light irradiation in the range of solution pH 2-10 was established.