This work demonstrates that the fungicide thiophanate-methyl generates a sensitizing photoproduct upon irradiation in solar light and explains how the reaction proceeds. The degradation profileof aqueous thiophanate-methyl in simulated solar light showed an autoaccelerated shape indicating the formation of some photodegrading compounds. Among the detected photoproducts, I (λmax = 340 nm), a quinoxaline derivative generated in the very beginning of the reaction seemed to be the molecule responsible for this autoacceleration. Further experiments were conducted to confirm this hypothesis. The laser flash photolysis (λexc = 355 nm) of I generated a transient species reminiscent to the triplet excited state of unsubstituted quinoxaline. The two triplets reacted with the phenolic probe, 2,4,6-trimethylphenol, with k = 2 × 109 M−1 s−1. Steady-state irradiations showed that thiophanate-methyl drastically accelerated the photolysis of the probe in an auto-accelerated reaction confirming the production of a sensitizing photoproduct. Moreover, unsubstituted quinoxaline sensitized the degradation of thiophanate-methyl showing that this latter contains labile H atoms. All these data confirmed that I was the sensitizer responsible for the auto-accelerated light induced transformation of thiophanate-methyl. The mechanism of formation of I was studied by quantum calculations. It proceeds through photocyclisation to form a SS bridge, followed by intra and intermolecular H abstractions and finally departure of HSS.The sensitizing properties of micropollutants photoproducts open a large array of investigations. Our study provides a reliable approach to investigate the overlooked photosensitizing effect of photoproducts during photochemical reactions in water and opens the way to a vast array of research.