Sonolysis, enzyme treatment, and a combination of the two processes were tested for the degradation of phenol in aqueous medium. With sonolysis alone, 423 kHz ultrasound eliminated phenol molecules through hydroxyl radical reactions. In the presence of hydrogen peroxide, the enzyme horseradish peroxidase (HRP) catalyzed the oxidation of phenol to insoluble polymers which could be separated easily from the solution. In the combined system (HRP, H2O2, US-423 kHz) the degradation rate was accelerated, and the color of the solution changed to dark brown without any precipitate formation. The enhancement was more significant in the presence of additives such as polyethylene glycol (PEG). The higher rate induced through the use of ultrasound leads to the reduction of HRP dose requirements, which is of importance for the economic feasibility of this method. The enzyme solution alone did not show any inactivation up to 45 min sonication at 423 kHz. Investigation of phenol removal efficiency was mainly focused on enzyme, phenol and PEG concentrations, as well as the sequence and method of addition of reagents. (C) 2004 Published by Elsevier B.V.