Photochemical decomposition of a fluorotelomer unsaturated carboxylic acid, C 3 F 7 CFCHCOOH (1), in the presence of WO 3 and an electron acceptor (S2O82- or H 2 O 2 ) in water under visible-light irradiation was investigated. Under an O 2 atmosphere, 1 was not decomposed either by TiO 2 (P25) or WO 3 alone. A combination of WO 3 and H 2 O 2 also resulted in almost no decomposition of 1. In contrast, irradiation in the presence of a combination of WO 3 and S2O82- (potassium salt) efficiently decomposed 1 to F − , CO 2 , C 3 F 7 COOH, and C 2 F 5 COOH. The decomposition of 1 was affected by the counter cation of S2O82-: the decomposition extent was higher with K 2 S 2 O 8 than with (NH 4 ) 2 S 2 O 8 . The decomposition of 1 was further enhanced when the reaction in the presence of WO 3 and K 2 S 2 O 8 was carried out under an argon atmosphere. Under O 2 , the amount of H 2 O 2 formed in the reaction solution was an order of magnitude higher than the amount formed under argon. This fact suggests that the decrease in the decomposition of 1 under O 2 can be ascribed to the formation of H 2 O 2 , which consumed S2O82- and SO4-.