Ozonation is one of the common methods for pollutant removal and disinfection in water treatment. The reaction between the ozone and organic pollutants involves complex oxidation pathway which may lead to the formation of various ozonation by-products. In some cases, toxic ozonation by-products may be produced. In this study, the ozonation by-products of acebutolol, one of the common B-blockers, were identified. The mechanisms for the transformation of acebutolol during ozonation were elaborated. In this study, ozonation was carried out at pH 2 (with the presence of radical scavenger), 7 and 12, in order to study the role of the ozone (O-3) and hydroxyl radical ((OH)-O-center dot) in the transformation of acebutolol. Structure elucidation of the ozonation by-products was carried out using HPLC coupled with quadrupole time-of-flight high resolution mass spectrometry. Sixteen ozonation by-products were identified, of which fifteen have never been reported elsewhere. Based on the detected ozonation by-products, acebutolol can be degraded by both O-3 and (OH)-O-center dot. When the O-3 itself becomes a reactive species, the transformation of the acebutolol proceeded via the reaction between the O-3 and the aromatic ring of the acebutolol to form the aromatic ring hydroxylated and aromatic ring opening by-products. Without the radical scavenger, ozonation at pH 7 and 12 showed that the transformation of the acebutolol proceeded via the involvement of the (OH)-O-center dot and different aliphatic chain degraded and hydroxylated by-products were detected. (C) 2014 Elsevier B.V. All rights reserved.