It has been experimentally reported that not only oxidation reactions but also reduction reactions occur in aqueous solutions under ultrasound without any additives. According to the numerical simulations of chemical reactions inside an air or argon bubble in water without any additives under ultrasound, reducing agents produced from the bubbles areH,H2,HO2(which becomes superoxide anion (O2−) in liquid water),NO, andHNO2(which becomesNO2−in liquid water). In addition,H2O2sometimes works as a reducing agent. As the reduction potentials ofHandH2(in strongly alkaline solutions forH2) are higher than those ofRCHOHradicals, which are usually used to reduce metal ions,HandH2generated from cavitation bubbles are expected to reduce metal ions to produce metal nanoparticles (in strongly alkaline solutions forH2to work). It is possible that the superoxide anion (O2−) also plays some role in the sonochemical reduction of some solutes. In strongly alkaline solutions, hydrated electrons (e−aq) formed fromHatoms in liquid water may play an important role in the sonochemical reduction of solutes because the reduction potential is extremely high. The influence of ultrasonic frequency on the amount ofHatoms produced from a cavitation bubble is also discussed.