The interaction of xenon with copper⁄6-hydroxydopa (2,4,5-trihydroxy-phenethylamine) quinone (TPQ) amine oxidases from the plant pulses lentil (Lens esculenta) and pea (Pisum sativum) (seedlings), the perennial Mediter-ranean shrub Euphorbia characias(latex), and the mammals cattle (serum) and pigs (kidney), were investigated by NMR and optical spectroscopy of the aqueous solutions of the enzymes.