The concentration of the N-aminomethylated benzotriazole copper passivator Irgamet 39 in transformer oil (2) plummets on severe oxidation. The phenomenon was determined to be independent of oxidation products such as aldehydes, ketones, esters, or carboxylic acids. Hydroperoxides were determined to have a profound effect on the measured degradation of 2. This is also true for tolylbenzotriazole (TTA, compound 1b). A mechanism of decay mediated by hydroxyl radical addition to benzotriazoles was proposed where the reaction of intermediate diazonium salts with nucleophiles destroys the triazoles. The rate of disappearance of 2 was determined to be proportional to the concentration of 2 and the square of the concentration of added hydroperoxide in a pure oil-like hydrocarbon solvent (PAO). The rate in a transformer oil would be very low, because of the presence of antioxidants (sulfur compounds or phenolic antioxidants). Very low peroxide concentrations were observed in transformer oils.