The kinetics and the mechanism of degradation of the mesotrione herbicide by three electrochemical advanced oxidation processes (EAOPs) systems, namely electro-Fenton (EF) with Pt anode (EF-Pt), anodic oxidation with BDD anode (AO) and EF-boron doped diamond anode (EF-BDD), were investigated in acidic aqueous solutions. The degradation of mesotrione obeyed apparent first-order reaction kinetics, and its absolute rate constant value with hydroxyl radicals at pH 3.0, determined by the competitive kinetics method, was found to be 8.20 × 10⁸ L mol^–1 s^–1. Fourteen different mesotrione degradation products were separated and/or identified using HPLC / MSⁿ analyses. A rationalized scheme was proposed for the reaction pathways of mesotrione degradation in the EF process. The mesotrione mineralization yield values ranged between about 58 and 97% for a 6-h electrolysis time, according to the type of EAOP system and the electrolysis current intensity value. The evolution of the toxicity of mesotrione aqueous solutions with electrolysis time (t) was investigated during treatment by the EF-Pt and EF-BDD systems. Toxicity was measured with the Microtox® method, based on the luminescence inhibition of marine bacteria Vibrio fischeri. The curves % inhibition vs. time were found to depend on the toxicity of formed mesotrione degradation products.