Various organic compounds, such as butyric acid, dodecylbenzenesurfonic acid, chlorophenols, hydroxybenzoic acids, etc., were decomposed by ultrasonic irradiation (200kHz, 200W) from the viewpoints of wastewater treatment and to determine the reaction kinetics. The rates of degradation changed by the types of organic compounds. In the case of butyric acid, the rates of degradation were dependent on the pH of the irradiated solution: the rate was faster in the acid solution than in the neutral solution. This result was elucidated by the different hydrophobic intensity of butyric acid which was related to the proton dissociation of butyric acid. In the cases of aromatic organic compounds, the rates of degradation were also different among the compounds. The rates did not relate to the reaction rate constants. Taking into account unique reaction sites of cavitation bubbles, we discussed the relation between the physicochemical properties of organic compounds and the rates of sonochemical degradation. It was suggested that the hydrophobicity of organic compounds is one of the most important factors to determine the rates of degradation.