Perfluorosulfonic acids (PFSAs) are chemical compounds of the formula C<sub>n</sub>F<sub>(2n+1)</sub>SO<sub>3</sub>H and thus belong to the family of perfluorinated and polyfluorinated alkyl compounds (PFASs). The simplest example of a perfluorosulfonic acid is the trifluoromethanesulfonic acid. Perfluorosulfonic acids with six or more perfluorinated carbon atoms, i.e. from perfluorohexanesulfonic acid onwards, are referred to as long-chain.
Perfluorosulfonic acids are organofluoroanalogues of conventional alkanesulfonic acids, but they are several pK<sub>A</sub> units stronger (and are therefore strong acids). Their perfluoroalkyl chain has a highly hydrophobic character.
Perfluorooctanesulfonic acid, for example, has been used in hard chromium plating. Since the early 2000's has been used as a replacement for perfluorooctanesulfonic acid.
Perfluorooctanesulfonic acid was included in Annex B of the Stockholm Convention in 2009 and subsequently in the EU POPs Regulation.
Perfluorohexanesulfonic acid was included in Annex A of the Stockholm Convention in 2022.