To investigate the hydroxyl radical (OH·) formation during ozonation, OH· yields (Φ) in the reactions of O₃ with O₃ reactive compounds such as electron-activated aromatic compounds and amines were determined by the “tBuOH· assay”. In addition, the Φ values in the ozonation of waste water effluent, surface water and groundwater were investigated. The results showed that the Φ values in the reactions of aromatic compounds, amines and olefins with ozone were in the range of 10％-30％, 15％-35％ and 0, respectively. The Φ value was about 41％ in the reaction of hydroxyl ion (OH-) with ozone. Competition of OH- with water matrix for O₃ consumption resulted in lower Φ value in waters with higher content of DOC. Generally, the Φ values were in the trend of groundwater> surface water> waste water effluent.