We show that the main photochemical processes involved in the phototransformation of anionic ibuprofen (IBP) in surface waters are the reaction with *OH, the direct photolysis and possibly the reaction with the triplet states of chromophoric dissolved organic matter ( 3CDOM*). These conclusions were derived by use of a model of surface water photochemistry, which adopted measured parameters of photochemical reactivity as input data. The relevant parameters are the polychromatic UVB photolysis quantum yield (Φ IBP = 0.33 ± 0.05, μ±σ), the reaction rate constant with *OH (k IBP,*OH=(1.0 ± 0.3){dot operator}10 10 M -1 s -1), the 1O 2 rate constant (kIBP,1O2=(6.0 ± 0.6){dot operator}10 4 M -1 s -1), while the reaction with CO3-* can be neglected. We adopted anthraquinone-2-sulphonate (AQ2S) and riboflavin (Ri) as CDOM proxies and the reaction rate constants with the respective triplet states were k IBP,3AQ2S*=(9.7 ± 0.2){dot operator}10 9 M -1 s -1 and k IBP,3Ri* = 4.5{dot operator}10 7 M -1 s -1. The reaction with 3CDOM* can be an important IBP sink if its rate constant is comparable to that of 3AQ2S*, while it is unimportant if the rate constant is similar to the 3Ri* one. The photochemical pathways mainly lead to the transformation (oxidation and/or shortening) of the propanoic lateral chain of IBP, which appears to be significantly more reactive than the isobutyl one. Interestingly, none of the detected intermediates was produced by substitution on the aromatic ring.