The dissociation of water molecules by ultrasound irradiation under aerobic conditions was demonstrated experimentally. To be able to detect the dissociation of water molecules, we performed the ultrasound irradiation of O-17-labelled water ((H2O)-O-17) under aerobic conditions. The hydroxyl and hydrogen radicals generated during the ultrasound irradiation process were trapped with 5,5-dimethyl-1-pyrroline- N-oxide (DMPO), and electron spin resonance (ESR) spectroscopy was performed on the DMPO spin adducts. In the ESR spectrum, a 15-line signal attributable to the trapping of the hydroxyl radicals containing O-17 ((OH)-O-17 radicals) by DMPO together with a 4-line signal attributable to the trapping of the hydroxyl radicals containing O-16 ((OH)-O-16 radicals) by DMPO were observed. The generation of (OH)-O-17 radicals indicated that (H2O)-O-17 was dissociated by the sonolysis process under aerobic conditions. On the other hand, the ESR signal attributable to the trapping of hydrogen radicals by DMPO was not observed, suggesting that hydrogen radicals were not generated during the dissociation of water molecules. (C) 2017 Elsevier Inc. All rights reserved.