The concept of solute hydrophobicity has been of extensive use in environmental chemistry to estimate the extent of sorption or partitioning of organic contaminants from water to nonaqueous phases such as natural organic matter, carbon surfaces, and biological lipids. For neutral organic compounds, the octanol/water partition coefficient (Kow) has been considered as a scale of solute hydrophobicity because Kow correlates well with various relevant partition coefficients, including soil/sediment organic carbon/water (Koc) and biological phospholipid membrane/water (Klipw) partition coefficients on the log-log scale, particularly when only a specific set of compounds with similar interaction properties (e.g., nonpolar aromatic compounds) are considered. For this reason, Kow is accepted or being introduced as chemical assessment criterion to account for the environmental mobility and bioaccumulation potential of chemicals in regulations such as the EU CLP and REACH regulations and the Stockholm Convention. However, for ionic compounds, i.e., compounds carrying one or more charges, the equilibrium concentration ratio between octanol and water highly depends on the coexisting electrolyte concentrations, and thus the Kow of ionic compounds cannot be defined in isolation from the experimental conditions. 9,10 Moreover, even if Kow at a specific condition is chosen as a reference, the bulk partition of ionic compounds between octanol and water may not represent their actual environmental and biological partition mechanisms and thus such Kow does not serve as a useful metric. Per-and polyfluoroalkyl substances (PFAS) have been of high environmental concern due to their recalcitrance to degradation. PFAS are a diverse group of chemicals with perfluorocarbon atoms and include a number of ionic compounds. For example, PFAS of highest concern to date, such as perfluorooctane sulfonate (PFOS) and carboxylate (PFOA) and many of their industrial substitutes, are anionic. Capturing sorption and partition properties for legacy, emerging, and premarket anionic PFAS is a prerequisite for their environmental fate assessment but is not possible by using Kow for the reasons stated above.