Utilizing a pivalamidoethyl-based model for the 2,4,5-trihydroxyphenylalaninequinone cofactor of copper amine oxidases, we recently reported (J. Am. Chem. Soc. 1995, 117, 3096) catalytic aerobic deamination of activated amines and full characterization of the various possible intermediates which could be involved in either a transamination mechanism, employed by the enzymes, or an alternate addition-elimination mechanism shown to compete in other quinone model reactions. However, attempts to distinguish between these two mechanisms through product analysis under anaerobic single-turnover conditions were thwarted by the occurrence of redox interchange reactions that scrambled the initial cofactor reduction product. Utilizing a tert-butyl cofactor model, we here describe reaction conditions which permit a definitive conclusion of transamination in the case of benzylamine using either CH3CN or DMSO as solvent. The interplay between the rate-limiting step in such conditions and the appearance of a primary kinetic isotope effect using PhCD(2)NH(2) is discussed. Further, the use of 5-amino-2,4-cyclohexadienecarboxylic acid permits unambiguous mechanistic conclusions because the initial single-turnover products in this case tautomerize to aromatic moieties incapable of redox interchange. The reaction follows mainly transamination, though addition-elimination appears to compete somewhat for this branched primary amine.