This study utilized 20 kHz ultrasonic irradiation as a mechanism of hydrogen peroxide production. The effects of various operating parameters were investigated, including ultrasonic amplitude, solution pH, source of water, initial dibutyl phthalate concentration and the presence of hydrogen peroxide. During the irradiation, the H2O2 concentration arising was monitored. The results indicate that H2O2 is produced by cavitation during ultrasonic irradiation. An increase in ultrasonic amplitude increases the amount of hydrogen peroxide produced. The initial pH of the solution does not affect the efficiency of processes substantially. H2O2 is regarded as one of the most effective additives enhancing the sonochemical production of hydroxyl radicals, but too high a dose is known to exert a negative effect. Above a 0.1 mu M dose of H2O2, the amount of H2O2 formed decreased as the concentration of H2O2 increased. Thus, the concentration of hydrogen peroxide plays a crucial role in the extent to which effectiveness of the combined process is enhanced. The negative effect on reactions of the presence of additional components in the reaction solution was also confirmed. It was therefore concluded that experimental evaluation of optimum parameters of hybrid processes is a matter of importance.