Abstract Concentrations of aqueous-phase perfluorooctanoic acid (PFOA), a representative perfluorinated aliphatic (PFA) compound, are shown to be reduced effectively via reaction with horseradish peroxidase (HRP), hydrogen peroxide, and a phenolic cosubstrate (4-methoxyphenol). Reaction rate profiles are pseudo–first order, yielding an apparent best-fit removal rate constant of k1 = 0.003/min (r2 = 0.96, n = 14). Approximately 68% depletion of the parent compound and 98% depletion of its related acute aquatic toxicity are achieved in 6 h. Because no PFOA removal is observed in the absence of cosubstrate and/or following consumption thereof, we conclude that radical intermediate species generated during reaction between HRP and 4-methoxyphenol mediate nonspecific depletion of PFOA and that these intermediates may be sufficiently reactive to sever the extremely stable C–F bonds of PFOA. These results are consistent with measurements of reaction by-products, including fluoride ion and various aliphatic species of shortened chain length. Based on these findings, we conclude that PFA degradation may occur via one of two mechanisms: Kolbe decarboxylation followed by stepwise conversion of–CF2 units to CO2 and fluoride ion, or radical abstraction from a double bond with subsequent fragmentation. Our results indicate that under appropriate conditions, enzymatic degradation may comprise a natural transformation pathway for PFAs. Moreover, we anticipate that appropriately engineered enzymatic processes may hold promise for treatment of PFOA-contaminated waters. This, to the best of our knowledge, is the first report to substantiate the efficacy of HRP-catalyzed reactions for contaminant removal via degradative reactions versus polymerization reactions.