EPR spectra for the purple mixed-valence [Cu^1.5+...Cu^1.5+], S = 1/2, site (CuA) in nitrous-oxide reductase (N2OR) were obtained after insertion of either ⁶³Cu or ⁶⁵Cu or both ⁶⁵Cu and [¹⁵N]histidine. The spectrum of ⁶⁵Cu- and [¹⁵N]histidine-enriched N2OR improved the resolution of the Cu hyperfine lines, but no lines from nitrogen and proton couplings were resolved. The Cu hyperfine parameters obtained by a theory analogous to that of Maki and McGarvey were indicative of a highly covalent Cu site. The total Cu character (CuA1 + CuA2) in the ground state wave function required to describe the spin density distribution was 31-37% compared to 41% for type-1 Cu in plastocyanin. This value does not completely account for the reduction of gmax from 2.23 of type-1 Cu in plastocyanin to 2.18 of CuA. Remaining discrepancies were discussed in terms of different alignments of the principal axes for the hypothetical monomeric CuA1 and CuA2 in [Cu^1.5+...Cu^1.5+]. This effect appeared in the simulations of the EPR spectra as a noncoincidence between the Cu hyperfine and g principal axis systems. The g-value analysis of CuA predicts an electric dipole forbidden absorption band in the near-infrared region. Based on X-ray structural data of CuA in cytochrome c oxidase, iterative extended Hückel and UHF-INDO/S calculations on a sulfur-bridged (NH3)Cu^1.5+(SCH3)2Cu^1.5+(NH3)]⁺ core were used to interpret the EPR results. The ground state was selected as ²B3u and not ²B2u, because ²B2u has very little spin density (<0.5%) on the coordinated nitrogen atoms, which contradicts the experimental value of 3-5% spin density found on NHis.