Mass transfer was investigated in ultrasonic reactors by means of the electrochemical method (feri-ferro-cyanide couple, under diffusion limitations). This method allowed for the determination of the active zones in the reactor. The oxidation of a model pollutant, phenol, has been carried out in the previously screened reactor, with ultrasound alone and with ultrasound associated with electrolysis. With a 20 kHz sonication, the electrochemical oxidation of phenol in NaCl media allows the conversion of 75% of initial phenol within 10 minutes of treatment. However a toxic intermediate, p.quinone was formed. At 500 kHz a conversion of 95% of the initial phenol was obtained within the same treatment time, and final products of degradation were acetic and chloroacrylic acids.