The environmental risks of UV filters have attracted a lot of attention due to the increasing use of personal care products. Butyl methoxydibenzoylmethane (BMDBM) is one of the commonly used organic UVA filters and is widely detected in surface waters. Once in sunlit surface waters, photochemical processes including direct photolysis and indirect photolysis play key roles in its transformation. I will be presenting results from our investigations on the phototransformation of BMDBM during direct photolysis and hydroxyl radical (•OH) photooxidation. BMDBM in aqueous solutions were exposed to polychromatic light (300 nm to 575 nm) and UVB light (300 nm). H2O2 and NaNO3 were selected as the photolytic source of •OH. Ultrahigh performance liquid chromatography-quadrupole time-of-flight mass spectrometry (UPLC-qTOF/MS) was used for both BMDBM detection and transformation products (TPs) identification. Results showed that even under the conditions that •OH was closed to that of sunlit surface waters, BMDBM phototransformation was governed mainly by direct photolysis. Higher concentrations of •OH were needed to enhance the BMDBM photodegradation and TPs transformation. Kinetics data suggested that direct photolysis half-lives of BMDBM were affected by its initial concentration. It is inferred that the half-life of BMDBM at the level of ng/L in surface waters will likely be several days. This will be the reason that the photo-unstable BMDBM is frequently detected in surface waters. TPs were mainly formed from α-cleavage involving the keto form of BMDBM. Predictions on the potential ecotoxicity and migration possibilities of BMDBM and its TPs suggested that some of the TPs had higher predicted mobility and ecotoxicity than BMDBM. This study provides new information on the photochemical fate of BMDBM in sunlit surface waters, which will be useful for aquatic risk assessment studies.