The aqueous decomposition of trichloroethylene (TCE) oxide is shown to involve both pH-independent and hydronium ion-dependent regions. C-C bond scission is a major reaction at all pH values. Disappearance of TCE oxide is the rate-determining step fur the formation of CO under the conditions studied. The product distribution of CO and three carboxylic acids (HCO2H, Cl2CHCO2H, and glyoxylic acid) did not change considerably over the pH range of -1.5-14, in general, even though the hydrolysis mechanism changes from hydronium ion-dependent to pH-independent. Mechanisms for the hydronium ion-dependent and pH-independent hydrolysis of TCE oxide were elucidated on the basis of the results of (H2O)-O-18 and H incorporation and identification of products of the reaction of TCE oxide with lysine in both (H2O)-O-16 and (H2O)-O-18. In the pH-independent hydrolysis, a zwitterionic intermediate could be formed and undergo an intramolecular rearrangement (Cl- shift) to generate dichloroacetyl chloride, which would subsequently decompose to Cl2CHCO2H. The zwitterionic intermediate could also hydrolyze at the less sterically hindered methylene to give a glycol anion, which would dehydrohalogenate to form an oxoacetyl chloride intermediate. The oxoacetyl chloride could hydrolyze to generate either glyoxylic acid, as a final product, or an anionic intermediate, which could, go through a concerted mechanism to generate CO, HCO2H, and chloride. A mechanism proposed for the hydronium ion-dependent hydrolysis is very similar to that for the pH-independent hydrolysis except for the first step, which involves hydronium ion attack on TCE oxide to form a TCE-oxide cation intermediate. The lysine amide adducts were characterized by HPLC and mass spectrometry as those resulting from reaction with the postulated acyl chlorides.