Here we present the results obtained by applying a novel hybrid advanced oxidation technique, which we have called sonoelectro-Fenton (sono-EF) process. for the degradation of organic pollutants in aqueous medium. Its high performance arises from the coupling between ultrasound irradiation and the in situ electrogeneration of Fenton's reagent. An undivided electrolytic cell with a Pt anode and a three-dimensional carbon-felt cathode has been used to carry out the very effective electro-Fenton (EF) process at constant current, which allows the production of great amounts of hydroxyl radicals ((center dot)OH). The synergistic action of somication in the sono-EF process has been studied at low and high frequency within a range of output power. We demonstrate that the destruction of the herbicides 4,6-dinitro-o-cresol (DNOC) and 2.4-dichlorophenoxyacetic acid (2,4-D) is significantly accelerated, whereas no improvement is observed for the degradation of the dye azobenzene (AB), suggesting that the nature of the organic structure plays an important role. A pseudo-first order kinetics is found in all cases. Similar results are reached for sono-EF at low and high frequency, but the lowest ultrasounds power (i.e., 20 W) is shown to be much better. We conclude that the improvement yielded by sono-EF is due to various contributions, which in order of relevance are: (i) the enhanced mass transfer rate of both reactants (Fe(3+) and 02) towards the cathode for the electrochemical generation of Fenton's reagent (Fe(2+) + H(2)O(2)) and its transfer into the solution in order to enhance Fenton's reaction kinetics thus increasing the (center dot)OH production rate and accelerating the destruction process, (ii) the additional generation of (center dot)OH by sonolysis, and (iii) pyrolysis of organics due to cavitation generated by ultrasound irradiation. (c) 2008 Elsevier B.V. All rights reserved.