In this work, we examined the oxidative transformation of per-and polyfluorinated ether sulfonates (PFESAs) related to Nafion polymer production. Nafion membranes are extensively used in the chloralkali electrochemical process and fuel cells. Various "Nafion by-product" PFESAs have been detected in surface water, aquatic animals, and humans. However, these PFESAs with ether bonds and -CF3 branches (Figure ) cannot be detected by the current TOP assay. The findings from PFESAs oxidation are further compared with the extensively studied legacy PFAS to (1) reveal novel mechanistic insights and (2) add new tools for the TOP assay of emerging PFAS pollutants.