Liquid chromatography-mass spectrometry (LC-MS) analyses before light irradiation suggested that the commercial linear PFOS used in this study contained several branched isomers, such as perfluoroheptanesulfonate (PFHpS, m/z = 449), perfluoroheptanesulfonate (PFHxS, m/z = 399), and a small amount of PFOA (m/z = 413, Supplementary Fig. ). After light irradiation for 1 h, the signals associated with PFOS substantially decreased, and multiple peaks originating from molecules with one or two F atoms of PFOS replaced with H atoms were observed (m/z = 481 and 463, Supplementary Fig. ). In addition, several peaks associated with molecules, where several even numbers of F atoms were abstracted and some of C-C bonds were converted to double or triple bonds, were observed (such as m/z = 461, 443, 425, 407, 405). These results suggested that the H/F exchange reaction occurred during the decomposition of PFOS. On the other hand, only products with fewer than three substituted hydrogen atoms were observed by H/F exchange, and the most defluorinated anion detected even after prolonged light irradiation was C8F10H3SO 3-(m/z = 369). Moreover, the signals associated with the anions generated by H/F exchange reactions decayed with increasing irradiation time. Considering the near-unity defluorination efficiency in this system, another defluorination mechanism, i.e., the chain-shortening reaction likely occurred through visible-light irradiation. However, peaks derived from shorter-chain carboxylate anions, including trifluoroacetic acetate, were not observed even in the matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry (MALDI-TOF MS) measurements of the solution after the reaction (Supplementary Fig. ).