Mechanistic studies with 5-ethyl-3-methyllumiflavinium (Fl + )p erchlorate, ab iomimetic model for flavoenzyme monoamine oxidase B( MAO-B) catalysis, and the tertiary, allyl amine 1-methyl-4-(1-methyl-1 H-pyrrol-2-yl)-1,2,3,6-tetrahydropyridine (MMTP) revealt hat proton-coupled electron transfer (PCET) may be an important pathway for MAO catalysis. The first step involves as ingle-electron transfer (SET) leading to the free radicals FlC and MMTPC,t he latter produced by deprotonation of the initially formed and highly acidic MMTPC + .M olecular oxygen (O 2 )i sf ound to play a hitherto unrecognized role in the early steps of the oxidation. MMTP and severals tructurally similar tertiarya mines are the only tertiary amines oxidized by MAO, and their structural/electronic properties provide the key to understanding this behavior.Ageneral hypothesis about the role of SET in MAO catalysis, and the recognition that PCET occurs with appropriatelys ubstituted substrates is presented.