The degradation of malachite green (MG) in water by means of ultrasound irradiation and its combination with heterogeneous (TiO 2 ) and homogeneous photocatalysis (photo-Fenton) was investigated. Emphasis was given on the effect of key operating conditions on MG conversion and mineralization rates and the elucidation of major reaction by-products. Eighty-kilohertz of ultrasound irradiation was provided by a horn-type sonicator, while a 9W lamp was used for UV-A irradiation. The extent of sonolytic degradation increased with increasing ultrasound power (in the range 75–135W) and decreasing initial concentration (in the range 2.5–12.5mgL −1 ), while the presence of TiO 2 in the dark generally had little effect on degradation. Sonolysis under argon was substantially faster than under air, oxygen or helium leading to complete MG degradation after 120min at 10mgL −1 initial concentration and 135W ultrasound power. On the other hand, TiO 2 photocatalysis or photo-Fenton led to complete MG degradation in 15–60min with the rate increasing with increasing catalyst loading (in the range 0.1–0.5gL −1 for TiO 2 and 7–20mgL −1 for Fe 3+ ) and also depending on the gas used. TiO 2 sonophotocatalysis was always faster than the respective individual processes due to the enhanced formation of reactive radicals as well as the possible ultrasound-induced increase of the active surface area of the catalyst. For instance, the pseudo-first order rate constant for the sonophotocatalytic degradation at 0.5mgL −1 TiO 2 under air was 136.7×10 −3 min −1 with the respective values for photocatalysis and sonolysis being 112.6×10 −3 and 11.6×10 −3 min −1 . Irrespective of the process employed, mineralization was slower than MG decomposition implying the formation of stable by-products accompanied by the release of nitrates in the solution. GC/MS analysis verified the identity of primary intermediates and a reaction pathway based on them was proposed. Depending on the conditions employed, ecotoxicity of MG to marine bacteria was partly or fully eliminated.