The indirect photochemical degradation of mesotrione induced by humic substances was studied by excitation with a solar light simulator. The disappearance rate constant increased by increasing the concentration of molecular oxygen. Three main byproducts were observed. They all arise from the dissociation of the non- aromatic cycle. Collision-induced dissociation tandem mass spectrometry (CID-MS/MS) of the deprotonated molecule was carried out and the effect of the collision energy as well as the elemental compositions of the products ions was used to propose the chemical structures. The presence in the chemical structure of the SO2-Me moiety permitted several types of homolytic scissions leading to the formation of radical anions.