With a view to determining the parameters required to describe and to optimize sonochemical reactors, we have investigated gas-liquid transfer in a high frequency ultrasonic reactor. The effect of the sonicated volume and ultrasonic power on the dissolved dioxygen concentration in the reactor have been studied. Degassing action caused by cavitation bubbles and absorption caused by the acoustic fountain were shown. The study of the absorption phase leads to a model presented in this paper. The dissolved dioxygen concentration at equilibrium in the reactor which results from these two effects was shown to be constant in all cases.