The photodegradation of beta-blocker timolol in fulvic acid (FA) solution was investigated under simulated sunlight. The triplet excited state of FA ((3)FA*) and singlet oxygen (O-1(2)) were the main reactive species responsible for the degradation of timolol in the aerated FA solutions. Both dissolved oxygen and iodide ions (I-) are the efficient quenchers of (3)FA*. The photodegradation was drastically accelerated after removing the dissolved oxygen. The presence of I- inhibited the photosensitized degradation of timolol in the deoxygenated FA solutions, whereas the role of I- in the reaction was concentration-dependent in the aerated solutions. The other halide ions such as chloride (Cl-) and bromide (Br-) exhibited less effect on the photodegradation of timolol in both aerated and deoxygenated solutions. By LC-DAD/ESI-MS/MS analysis, the photoproducts of timolol in both aerated and deoxygenated FA solutions were identified. Electron transfer interaction occurred between (3)FA* and amine moiety of timolol, leading to the cleavage of C-O bond in the side chain and oxidation of the hexatomic ring. These findings suggest the photosensitized degradation was a significant pathway for the elimination of timolol in natural waters. (c) 2013 Elsevier B.V. All rights reserved.