Monoamine oxidases (MAOs) are flavoenzymes important in regulating amine neurotransmitter levels and are the central pharmacological targets in treating depression and Parkinson's disease. On the basis of quantum chemical calculations, we have proposed a new two-step hydride mechanism for the MAO-catalysed oxidative deamination of amines. In the rate-limiting first step, through its N5 atom, the flavin abstracts a hydride anion from the substrate α-carbon atom and forms a strong covalent adduct with the thus created cation. This is followed by flavin N1 deprotonation of the substrate amino group, facilitated with two active-site water molecules, to produce fully reduced flavin, FADH2, and neutral imine. We have demonstrated that our mechanism is in agreement with available experimental data and provided evidence against both traditional polar nucleophilic and single-electron radical pathways. These results provide valuable information for mechanistic studies on other flavoenzymes and for the design of new antidepressants and antiparkinsonian drugs.