Perfluorooctanesulfonate (PFOS) and related chemicals have been widely used as surface treatment agents, emulsifying agents and so on. Their high stability consequently results in environmental persistence and bioaccumulation. Techniques to decompose them to harmless species at stationary sources are desired. The method should involve cleavage of the C-F bonds to form F- ions, because a waste-treatment process for F- was already established. PFOS and other related chemicals can be efficiently decomposed to F- ions using iron and subcritical water. This method was also effective to PFOS contained in a coating agent used in a real manufacturing process.