X-ray absorption spectroscopy has been used to define the copper-site structures in both the resting (oxidized) and dithionite-reduced states of amine oxidases from bovine plasma, porcine plasma, porcine kidney, pea seedlings, and the gram-positive bacterium Arthrobacter P1. The Cu(II) EXAFS data are consistent with four first-shell N,O scatterers (three of which are imidazoles) at a distance of 1.98-2.00 Angstrom for all five amine oxidases. These scatterers are assigned as the equatorial ligands. Because the raw Cu(II) EXAFS data are essentially identical for all the enzymes examined, the crystallographically defined Cu(IJ) sites of the pea seedling and E. coli amine oxidases are excellent models for the copper sites in the mammalian enzymes. Analysis of the K-edges and EXAFS data for the reduced amine oxidases indicates that the Cu(I) sites are three-coordinate, with at least two imidazoles; the range for the bond distances among the reduced enzymes is 1.94-1.99 Angstrom. Therefore, the results are consistent with a decrease in coordination number from five (or, possibly six) to three upon reduction of Cu(II) to Cu(I). Three-coordinate Cu(I) complexes with predominately N,O ligands generally react with dioxygen, suggesting that the Cu(I) sites in amine oxidases map serve this role in catalysis.