There have been many reports on chemical reactions using ultrasound such as decomposition of polymer chains or initiation of polymerization by ultrasound. However, the mechanism of these sonochemical reactions has not been clarified yet. Understanding the mechanism will be useful for establishing a new radical polymerization system without an initiator, for new applications in the medical field, and so on. Decomposition of water (H[inf 2]O) into OH and H radicals is being studied by high-intensity ultrasound, because it is one of the simplest systems used for elucidation of the mechanism of sonochemical reactions. The influence of sonication conditions such as ultrasonic frequency, sonication time, and ultrasonic intensity on the production of OH radicals from water was studied, and the factors governing the sonolysis were examined. Since the OH radical is very short lived, DMPO (5,5-dimethyl-1-pyrroline N-oxide) was used as a scavenger, and the concentration of spin-trapped OH radicals was measured by ESR after sonication. The results show that (1) lower frequency ultrasound generates the radicals more efficiently, (2) the radical concentration initially increases linearly with time and the ultrasound intensity, and then (3) saturates at a value which is independent of the ultrasound intensity.