Spectrophotometry and rapid-scanning stopped-flow spectroscopy have been used to investigate the visible absorbance changes that occur in the course of the reduction of lentil (Lens esculenta) seedling amine oxidase by substrate. The catalytic cycle of the enzyme employs several intermediates but, owing to kinetic limitations, some of them were not identified in previous studies. In this study we have examined several substrates, either rapidly reacting (e.g. putrescine) or slowly reacting (e.g. gamma-aminobutanoic acid). Two forms of the enzyme, namely the Cu(I)-aminoresorcinol and quinone ketimine derivatives, whose characterization was elusive in previous studies, have been identified and assigned an optical spectrum. Moreover the reduced form of the enzyme is shown to be an equilibrium mixture of two species, the Cu(I)-semiquinolamine radical and Cu(II)-aminoresorcinol; these have been resolved by pH dependence and assigned spectra as well as a second-order rate constant for the reaction with oxygen. Thus the results presented here identify all the catalytic intermediates suggested by the chemical nature of the coenzyme and define their spectroscopic and reactivity properties.