In the present study, organophosphorus pesticides (OPs) (diazinon, methyl parathion, and parathion) were oxidized by bubbling ozone into a glass reactor. OP residues were detected using HPLC and ozonation intermediates were identified using GC-MS. The degradation of OPs followed pseudo-first-order kinetics through direct ozone oxidation and indirect hydroxyl radical oxidation. Diazinon, based on its relatively higher degradation constant, was easily degraded by ozonation. Increasing the pH of the solution accelerated diazinon degradation, but little effect was observed for methyl parathion or parathion. Diazoxon, methyl paraoxon and paraoxon were identified as ozonation intermediates of diazinon, methyl parathion and parathion, respectively. The ozonation of the PS group results in the formation of oxon intermediates, which suggests that OPs with this group would be degraded in a similar manner to that seen for the OPs tested in this study. Diazoxon was completely decomposed by ozonation in 30min, while trace methyl paraoxon and paraoxon accumulated to different amounts when the solution pH was varied. The presence of oxon intermediates should be noted in OP removal by ozonation.