Triclosan is an antimicrobial agent widely used in many household and personal care products. Widespread use of this compound has led to the elevated concentrations of triclosan in wastewater, wastewater treatment plants and receiving waters. In this study removal of triclosan by aqueous ozone was investigated and the degradation products formed during ozonation of an aqueous solution of triclosan were analyzed by GC-MS and HPLC-MS/MS. The following transformation products have been identified: 2,4-dichlorophenol, chloro-catecol, mono-hydroxy-triclosan and di-hydroxy-triclosan during treatment process. Cytotoxicity and genotoxicity of pure triclosan and 2,4-dichlorophenol have been investigated and the results showed reduced genotoxic effects after ozonation, though the respective chlorophenol is harmful to aquatic organisms.