Sevoflurane (fluoromethyl 2,2,2-trifluoro-[2,2,2-trifluoromethyl]ethyl ether) is a volatile anesthetic agent that is widely used in the U.S. and abroad. Sevoflurane undergoes degradation in the anesthetic circuit to form 2-(fluoromethoxy)-1,1,3,3,3-pentafluoro-1-propene (Compound A). As it is metabolized, Compound A alkylates the cysteine side chain in the tripeptide glutathione, which acts as a sort of scavenger for xenobiotics such as Compound A. The S-alkylated glutathione or glutathione S conjugate loses its C-terminal and N-terminal residues as it is further metabolized. This leaves a cysteine S conjugate of Compound A. The cysteine conjugate undergoes bioactivation by an enzyme known as β-lyase to produce nephrotoxic metabolites. Although Compound A is nephrotoxic in rats, Compound A-associated nephrotoxicity has not been observed in the human clinical use of sevoflurane, apparently because β-lyase activities are much lower in human kidney tissue than in rat kidney tissue. Since β-lyase reacts with carbonyl compounds by mechanisms involving deprotonation of the α-carbon, the reactions of Compound A-derived cysteine conjugates with the basic anionic species hydroxide, methoxide, and ethoxide were examined by Fourier-transform ion cyclotron resonance mass spectrometry. The anionic bases examined react with the cysteine conjugates by an initial deprotonation of the α-carbon to form an enolate intermediate followed by elimination of either a thiolate anion or of HF. Since the HF elimination leads to CF 2 loss, it is suggested that the F atom eliminated as HF comes from a CF 3 group. Collision-induced dissociation (CID) of the product ions suggested structures consistent with this overall mechanistic picture. It is evident from these results that the same mechanism by which other cysteine conjugates are bioactivated could operate in the case of Compound A. That is, deprotonation to form enolate intermediates could lead to the release of very reactive species that might inactivate enzymes by alkylating them or otherwise reacting irreversibly with them. The thiolate product could alkylate an enzyme under appropriate conditions. Condensed-phase hydrolysis of the thiolate product could produce 2-(fluoromethoxy)-3,3,3-trifluoropropionic acid, a known metabolite of Compound A. The HF loss channel produces not only HF, but also CF 2 , a very reactive species. Evidence is noted that a closely related enzyme substrate system reacts to release fluoride in condensed phases suggesting that the activation of a CF 3 observed here could be a more general process.