The sunlight filter benzophenone-4 (BP-4) is present in surface waters as two prevailing forms, the singly deprotonated (HA−) and the doubly deprotonated one (A2−), with pKa2 = 7.30 ± 0.14 (μ ± σ, by dissociation of the phenolic group). In freshwater environments, BP-4 would mainly undergo degradation by reaction with radical dotOH and direct photolysis. The form HA− has a second-order reaction rate constant with radical dotOH (kradical dotOH)(kOH) of (1.87 ± 0.31)*1010 M−1 s−1 and direct photolysis quantum yield Φ equal to (3.2 ± 0.6)*10−5. The form A2− has (8.46 ± 0.24)*109 M−1 s−1 as the reaction rate constant with radical dotOH and (7.0 ± 1.3)*10−5 as the photolysis quantum yield. The direct photolysis of HA− likely proceeds via homolytic breaking of the O-H bond of the phenolic group to give the corresponding phenoxy radical, as suggested by laser flash photolysis experiments. Photochemical modelling shows that because of more efficient direct photolysis (due to both higher sunlight absorption and higher photolysis quantum yield), the A2− form can be degraded up to 3 times faster than HA− in surface waters. An exception is represented by low-DOC (dissolved organic carbon) conditions, where the radical dotOH reaction dominates degradation and the transformation kinetics of HA− is faster compared to A2−. The half-life time of BP-4 in mid-latitude summertime would be in the range of days to weeks, depending on the environmental conditions. BP-4 also reacts with View the MathML sourceBr2−, and a rate constant View the MathML sourcekBr2−,BP−4=(8.05±1.33)*108M−1s−1 was measured at pH 7.5. Model results show that reaction with View the MathML sourceBr2− could be a potentially important transformation pathway of BP-4 in bromide-rich (e.g. seawater) and DOM-rich environments.