This work deals with the ultrasonic degradation (800 kHz) of crystal violet (CV) under different experimental conditions. The effects of saturating gas (argon, carbon dioxide and air), CV concentration (2.45-1225 mu mol L-1), pH (3-9) and power (20-80 W) were evaluated. The best performances were obtained at 80 W with argon as a saturating gas. The pH had no significant effect. The influence of several water matrices containing anions (chloride, sulphate and bicarbonate) and cations (Fe2+) on the sonolytic degradation of CV was also investigated. Significant differences were not observed with the presence of chloride and sulphate. However, at relatively low pollutant concentration (2.45 mmol L-1) bicarbonate showed a particular effect: a high bicarbonate concentration (350 mmol L-1) produced a detrimental effect, while a low bicarbonate concentration (3 mmol L-1) increased the efficiency of the process. The presence of Fe2+ (1 mmol L-1) also increased the CV (49 mu mol L-1) degradation by 32% after 180 min. Analyses of intermediates by GC-MS led to the identification of several sonochemical by-products: N,N-dimethylaminobenzene, 4-(N,N-dimethylamino)-4'-(N',N'-dimethylamino)benzophenone, and N,N,N',N'-tetramethy1-4,4'-diaminodiphenylmethane. The presence of these aromatic structures showed that the main ultrasonic CV degradation pathway is linked to the reaction with (OH)-O-center dot radicals. At the end of the treatment, these early products were converted into biodegradable organic by-products which could be easily treated in a subsequent biological treatment. (C) 2010 Elsevier B.V. All rights reserved.