Hydroxylamine oxidoreductase (HAO) from the autotrophic nitrifying bacterium Nitrosomonas europaea catalyzes the oxidation of NH2OH to HNO2. The enzyme contains eight hemes per subunit which participate in catalytic function and electron transport. The structure of the enzyme shows a unique spatial arrangement of the eight hemes, subsets of which are now observed in four other proteins. The spatial arrangement displays three types of diheme pairing motifs. At least four of the eight hemes are electronically coupled in two distinguishable pairs and one of these pairs is at the active site of the enzyme. Here, the use of quantitative simulation of the EPR signals allows determination of exchange couplings, and assignments of signals and reduction potentials to hemes of the crystal structure. The absence of any obvious heme-to-heme bonding pathway in the crystal structure suggests that the observed exchange interactions are derived from direct electronic overlap of porphyrin orbitals. This provides evidence for heme pairs which function as biological two-electron redox centers in electron-transfer processes.