Organic compounds in aqueous solution submitted to an ultrasonic irradiation behave differently according to their physical and chemical properties. In this work, degradation rate of phenol and carbon tetrachloride, has been studied at different frequencies, 20 kHz, 200 kHz, 500 kHz and 800 kHz. Whatever the frequency, it's easier to decompose CCl4 than phenol by means of ultrasonic waves. It is shown that the rates of reactions involving hydroxyl radicals (hydrogen peroxide formation and phenol degradation) have a maximal value at 200 kHz. The best yield observed at 200 kHz for the phenol degradation may be the result of a better HO° radicals availability outside of the bubble of cavitation. The degradation rate for carbon tetrachloride which decomposes into the bubble of cavitation increases with the frequency.