A nitroxide spin label (SL) has been used to probe the electron spin relaxation times and the magnetic states of the oxygen-binding heme–copper dinuclear site in Escherichia coli cytochrome bo 3, a quinol oxidase (QO), in different oxidation states. The spin lattice relaxation times, T 1, of the SL are enhanced by the paramagnetic metal sites in QO and hence show a strong dependence on the oxidation state of the latter. A new, general form of equations and a computer simulation program have been developed for the calculation of relaxation enhancement by an arbitrary fast relaxing spin system of S = 1/2. This has allowed us to obtain an accurate estimate of the transverse relaxation time, T 2, of the dinuclear coupled pair Fe(III)–CuB(II) in the oxidized form of QO that is too short to measure directly. In the case of the F' state, the relaxation properties of the heme–copper center have been shown to be consistent with a ferryl [Fe(IV)=O] heme and CuB(II) coupled by approximately 1.5–3 cm-1 to a radical. The magnitude suggests that the coupling arises from a radical form of the covalently linked tyrosine–histidine ligand to Cu(II) with unpaired spin density primarily on the tyrosine component. This work demonstrates that nitroxide SLs are potentially valuable tools to probe both the relaxation and the magnetic properties of multinuclear high-spin paramagnetic active sites in proteins that are otherwise not accessible from direct EPR measurements.