The oscillation of ultrasonic cavitation bubbles at various dissolved-air concentrations in a sonochemical reaction field is investigated experimentally by laser-light scattering. When a thin light sheet, finer than half the wavelength of sound, is introduced into the cavitation bubbles at an antinode of sound pressure, the scattered light intensity oscillates. The peak-to-trough light intensity is correlated with the number of bubbles that contribute to the sonochemical reaction. It is shown that as the dissolved air concentration is changed, the behavior of the spatial summation of the peak-to-trough intensity, determined by taking the degassing bubbles into account, is in good agreement with the behavior of the sonochemical reaction yield. Therefore, the present method of light scattering has proven to be a good cavitation-bubble detector for sonochemistry.