The sonochemical degradation rate of the charged substrates Acid Blue 40 (AB40) and methylene blue (MB) is enhanced by scavengers of hydroxyl radicals such as bicarbonate, carbonate, bromide, iodide and (only in the case of AB40) nitrite. No rate variation was observed with chloride, nitrate or sulphate, excluding a mere ionic strength effect. Oxidation of bicarbonate, carbonate, bromide, iodide and nitrite yields the corresponding radicals CO3 center dot-, Br-2(center dot-), I-2(center dot-) and (NO2)-N-center dot, reactive but less than (OH)-O-center dot. Degradation enhancement can occur if these radicals are sonochemically formed on the surface of the collapsing cavitation bubbles and undergo there radical-radical recombination at a lesser extent than (OH)-O-center dot. In this way the radicals would be more available than (OH)-O-center dot for substrate degradation, both at the bubble surface and in the solution bulk, which could more than compensate for their lower intrinsic reactivity. The varied reactivity toward different substrates of the sonochemically formed radical species could then explain why nitrite inhibits MB degradation while enhancing that of AB40. The sonochemical formation of Br-2(center dot-), I-2(center dot-) and (NO2)-N-center dot can give rise to halogenation and nitration in addition to oxidation processes. in fact bromo-, iodo- and nitrophenols were detected upon sonication of phenol in the presence of the corresponding anions, but only at quite elevated concentration values of nitrite, bromide or iodide (above 10 mM). Formation of harmful halogeno- and nitroderivatives could therefore take place on sonication of some wastewater rather than of typical natural waters. (C) 2007 Elsevier B.V. All rights reserved.