The study focuses first on the complexation of Cu II ions with mesotrione. Experimental absorption spectra combined with quantum chemical calculations highlight that Cu II ions bind to the molecule in bidentate mode between O1 and O7 atoms. The calculated electronic transitions of the complex show that three of them are mostly ligand-to-ligand electron transfer while the lowest-energy one involves a ligand-to-metal charge transfer (LMCT). The second aim of this work is to explore the effect of Cu II complexation on the photo-degradation of mesotrione. The kinetics and mechanisms of the reactions are proposed on the basis of the experimental results. The effect of different parameters such as the irradiation wavelength, the oxygen concentration and the pH of the mixture and also the production of reactive species such as hydroxyl radicals is investigated. The photochemical degradation of mesotrione in water arises from the triplet excited state of the molecule and results in a photo-hydrolysis process. For the complex, by exciting mainly in its LMCT absorption band, an electron transfer between copper and mesotrione probably occurs and the mechanism of mesotrione-CuII photo-degradation differs: in addition to the photo-hydrolysis route, a photo-cyclisation process takes place and the oxidation of the ligand is accompanied by the reduction of Cu II ions.