The degradation of trace amounts of phenol pollutant in water in presence of Zinc oxide catalyst under Ultraviolet (UV), Ultrasonic (US) and combination of UV and US irradiation has been investigated and optimum parameters for the mineralization of the pollutant have been identified. ZnO is more active than TiO2 for the sonophotocatalytic (UV + US) degradation of phenol. The extent of sonophotocatalytic degradation was higher compared to the individual sono or photocatalytic degradation or even their sum, thereby demonstrating a synergistic effect. The effect of different parameters, viz, dosage and particle size of the catalyst, concentration of pollutant, volume of reaction system, pH, US power density and presence of anions on the degradation rate has been evaluated. The degradation follows variable kinetics depending on the concentration of the substrate. The degradation is facilitated at lower reaction volume and acidic pH. The concentration of hydrogen peroxide formed in the reaction is fluctuating with time, indicating simultaneous formation and decomposition. Anions such as chloride and sulphate inhibit the sonophotocatalytic activity of ZnO. The kinetics and mechanism of the process and possible reasons for the synergy are presented. (C) 2012 Elsevier B.V. All rights reserved.