Formetanate hydrochloride (FMT) is an insecticide-acaricide widely used in agriculture. The sonolytic, photocatalytic and sonophotocatalytic degradations of FMT were studied in the presence of homogeneous (Fe3+) and heterogeneous (TiO2) photocatalysts using 213 kHz ultrasound. The degradation rates obtained for sonolysis, photocatalysis and sonophotocatalysis using TiO2 (1 g/L) were about 5.1, 32.6 and 28.1 x 10(-7) M min(-1), respectively. The combination of sonolysis with TiO2 photocatalysis showed a negative effect of combining two processes with a synergy index of 0.7. The degradation rates obtained for US + Fe3+, UV + Fe3+ and US + UV + Fe3+ are 14.1, 10.1 and 40.9 x 10(-7) M min(-1), respectively. Unlike TiO2, the Fe3+ addition during the sonophotocatalysis process was found to be beneficial with a synergy index of 1.6. The total organic carbon analysis showed that the mineralization process was synergistic by combining the two processes in the presence of Fe3+. Further studies with electrospray mass spectrometry analysis revealed the formation of mono and dihydroxylated derivatives of FMT during sonolysis. (C) 2010 Elsevier B.V. All rights reserved.