Tonalide was extensively used as fragrances in various personal care products and recognized as emerging contaminant recently. In this study, the transient reactions of hydrophobic tonalide with hydroxyl radical (center dot OH), one of highly reactive oxygen species (ROSs), was firstly investigated in acetonitrile through direct observation using laser flash photolysis. Two transient intermediates were formed with the peak centered at 360 and 450 nm, and the bimolecular reaction rate constants of tonalide with center dot OH were determined in both water and acetonitrile solutions with values of 4.04 x 10(9) and 1.95 x 10(9) M-1 respectively. Then, the photocatalytic experiments in a heterogeneous TiO2 system were carried out to study the decomposition kinetics of tonalide with "OH, which was confirmed as the dominant ROSs in photocatalytic system. Five degradation products were identified with molecule weight of 260, 272 and 274. Finally, the potential risks of reaction products formed through the reactions of tonalide with center dot OH were further investigated. It is found that the toxicity towards luminescent bacterium (Photobacterium phosphoreum) was not changed so much, and the estrogenic activity increased a little bit during the degradation of tonalide by center dot OH. (C) 2016 Elsevier B.V. All rights reserved.