Degradation of phenol in water was carried out by ultrasonic irradiation in the presence of the composite particles of titanium dioxide (TiO2) and activated carbon. Degradation of phenol as well as adsorption of phenol on the particles occurred at the same time. The amount of adsorbed phenol on the particles was estimated, and the overall phenol concentration in the system was calculated by adding the amount of adsorbed phenol on the particles and that dissolved in the liquid. The degradation rate in the presence of the composite particles was higher than that when the TiO2 and activated carbon were separately suspended. A kinetic model for degradation of phenol took into account the degradation both in the water phase and on the particle surface. The calculated results explained the effects of the amount of the particles and the TiO2 ratio in the particles on the degradation rate.