The magnetic properties of deoxyhemerythrin azide have been measured by combining multifield saturation magnetization data and multifrequency EPR spectra. The exchange coupling [J = −3.4 (1) cm−1; JS1•S2], average zero-field splitting parameters [|D1| = |D2| = 12 (1) cm−1 and E1/D1 = E2/D2 = 0.3], and average g values [g1 = g2 = 2.23 (5)] have been determined assuming identical iron sites with parallel zero-field splitting axes for the ferromagnetically coupled high-spin diferrous center. The observed ratio of the exchange coupling to the zero-field splitting (J/Di = 0.28) lies in the weak exchange coupling regime of a perturbation treatment of the full 25 × 25 spin Hamiltonian of the coupled diiron center. Perturbation expressions required for the description of the integer-spin EPR signals of two S = 2 metals in either the strong (J/Di > 1) or weak (J/Di < 1/3) exchange coupling regimes are presented. For deoxyhemerythrin azide the EPR signal arises from the lowest pair of energy levels of the coupled system and can be interpreted in either coupling regime. The saturation magnetization family of curves, on the other hand, are a thermal average of all levels populated from 2 to 200 K and can resolve this ambiguity in the interpretation of the EPR data. An excellent fit to the entire family of multifield saturation magnetization curvss has determined both the exchange coupling and zero-field splitting of deoxyhemerythrin azide, which lies in the weak coupling regime. In addition, quantitative agreement between saturation magnetization, integer-spin EPR, and optical data has been obtained.