The application of ozone along with hydrogen peroxide, commonly referred to as peroxone oxidation, is a widely investigated technique for wastewater treatment. Degradation of ozone in water is a key step in the pollutant degradation mechanism, particularly in peroxone oxidation. However, the degradation of ozone in water is not understood at a low pH (<6). This study reveals that current ozone degradation models overestimate degradation at a low pH because the rate constants involved in the dissociation equilibrium of the hydroperoxyl radical are inaccurate. Here, the rate constants of forward and backward reactions were calculated with ab initio quantum chemical calculations computed from the CCSD (T) theory to be 1.45 X 10(3) s(-1) and 8.6 X 10(7) m(3) kmol(-1) s(-1), respectively. After modifying the current kinetic model by using the calculated rate constants, the predictions of ozone half-lives at a low pH (<6) are improved by 1-2 orders of magnitude in pure water (without organic matter and carbonate species) in comparison with the available experimental results. The ozone decomposition kinetic model was used to develop a comprehensive kinetic model for peroxone oxidation of toluene. The results demonstrate that the new rate constants considerably improve the peroxone oxidation process as well.