The influence of pH on the degradation of hydrazine with a concentration of 0.1 mmol/L was investigated under the stirring (300 rpm) and ultrasonic irradiation conditions (200 kHz, 200 W) in the pH range of 1-9. It was found that the hydrazine degradation depended greatly upon pH under the ultrasonic irradiation condition, while it did not take place over the whole pH range under the stirring condition. Although it has been known that OH radicals and hydrogen peroxide are sonochemically formed from water, it was considered that the OH radicals played an important role of the hydrazine degradation, but not hydrogen peroxide. The pH dependence of the hydrazine degradation was discussed in terms of the relationship between the chemical structure and the basic dissociation constants of hydrazine. (C) 2007 Elsevier B.V. All rights reserved.