Most advanced oxidation processes (AOP) readily decolorize but are unable to mineralize aqueous azo dye solutions. The extent of mineralization-measured as total organic carbon (TOC) losses-during the 500-kHz sonication of azobenzene or methyl orange solutions increases from 20% to more than 80% in the presence of O-3 The abatement of the total organic load by the joint action of ultrasound and O-3 amounts to chemical synergism. Since TOC losses are not enhanced by ozonation followed by sonication and ground-state O atoms-that are produced by sonochemical O-3 thermolysis-are relatively unreactive, synergism likely involves the fast oxidation by ozone of free radical or unsaturated species generated by . OH radical attack on otherwise refractory products. Some of these products probably are saturated mono- and dicarboxylic acids, known to be resistant to O-3 oxidation. Nitrobenzene and benzoquinone, two rather persistent byproducts of sonolysis, are rapidly and completely mineralized by the combined oxidation treatment. Thus, direct ozonation of unsaturated sonolytic byproducts also accounts for part of the observed enhancement of the extent of mineralization. The anomalous kinetic behavior of the sonochemical degradation of benzoquinone (in the absence of O-3) is accounted for by its particularly high reactivity toward the relatively inert HO2. and O-2(-). radicals.