Three β,β,β′,β′-2H4-labelled α,ω-diamines were synthesized and incubated with pea seedling diamine oxidase. The aminoaldehydes formed by oxidative deamination cyclized to the corresponding imines, which were trapped with benzoylacetic acid. In all three cases the acetophenone derivatives produced were shown by NMR and mass spectrometry to contain four deuterium atoms. The retention of all four deuterium atoms demonstrates that oxidative deamination of α,ω-diamines catalyzed by pea seedling diamine oxidase does not involve an enamine intermediate as previously suggested.