Photodegradation process is one of the most important abiotic factors affecting the fate of pesticides in aquatic ecosystems. As consequence of this transformation pathway, different photoproducts can be formed with unknown toxicity, mobility and persistence in aqueous media. The information obtained is even more valuable for new families of pesticides, like cyclohexanedione herbicides, because the scientific literature about their environmental behavior and persistence is quite limited. Although there are evidences that the persistence of sethoxydim, a cyclohexanedione herbicide, in aquatic media is largely affected by reactions initiated by solar radiation, there are no studies about the photoproducts formed in natural waters. Therefore, we studied for the first time the photoproducts formed during the irradiation of sethoxydim herbicide under natural and simulated sunlight in different sources of waters. Ten photoproducts of sethoxydim were separated and identified by means of the HPLC-ESI-Qtof-MS technique. Main phototransformation processes of sethoxydim involved isomerization of oxime ether bond, oxidation of sulfur atom and cleavage of oxime ether bond. Photoproducts dealkoxy ketone, dealkoxy sulfoxides and dealkoxy sethoxydim resulted to more photostable than sethoxydim hence probably more persistent in the environmental. Both radiation and water sources influenced the concentrations of photoproducts formed during the irradiation of sethoxydim.These findings highlight the need for further insight in the study of photoproducts to understand the environmental fate of sethoxydim herbicide.