The thermodynamic stability of the 89 isomers of the eight-carbon-atom compound perfluorooctane sulfonate (PFOS) in their neutral and anionic forms has been studied in the gas phase, n-octanol, and water using density functional theory (B3LYP/6-311+G(d,p)). The gas-phase calculations are compared with previous semiempirical and partial ab initio studies; the calculations in water and n-octanol are reported for the first time. The results obtained indicate that the thermodynamic stability assessment of this family of persistent organic pollutants is independent of the environment and type of species (neutral or anionic) considered and that it is important to consider other PFOSs outside of the 83-89 set, which is the most frequently studied.