Decomposition of poly(tetrafluoroethylene-co-hexafluoropropylene) copolymer (FEP) in supercritical and subcritical water was investigated with the aim of waste treatment. The efficiency of such decomposition was examined by using either oxidizing agents (O 2 , H 2 O 2 ) or an alkaline reagent (KOH). When O 2 was chosen, the highest F -and CO 2 yields, 75% and 64%, respectively, were obtained from a reaction of FEP in supercritical water at 384 °C for 24 h. Under these conditions, traces of CHF 3 were detected in the gas phase, the amount of which decreased with increasing the reaction time. H 2 O 2 gave slightly higher reactivity than O 2 . In contrast, reactions with KOH induced efficient fluorine mineralization. When FEP reacted in 3.0 M KOH solution at 360 °C for 18 h, the F -yield reached 98%. Hence, complete fluorine mineralization was achieved, where very little CO 2 (~0% yield) and no CHF 3 were generated in the gas phase. Furthermore, 19 F-NMR spectroscopy and combustionion chromatography revealed that the reaction solutions did not contain any organofluorine compounds during the reactions. FEP decomposed, releasing F -into the reaction solution, resulting in the formation of amorphous carbon in the residue.