The difference between the magnetic susceptibilities of native and thionine-oxidized molybdenum-iron protein from Azotobacter vinelandii nitrogenase was measured by the nuclear magnetic resonance method. Reversible oxidation of the MoFe protein by 4 to 8 electron eq of thionine/mol made the protein more paramagnetic than it was in the native state. The NMR susceptibility results were analyzed in terms of a model for the spin states of the iron centers in the MoFe protein based on low temperature electron paramagnetic resonance and Mössbauer spectral studies. The model proposes that the native protein contains 2 "M" centers (S = 3/2) and 4 "P" centers (S = 0)/mol and that the oxidized protein has diamagnetic M centers and paramagnetic P centers with S greater than or equal to 3/2. Assuming that this model holds at 280 K, the NMR susceptibility results show that the effective magnetic moment of the oxidized P centers is larger than that of the native M centers. Based on an analysis in terms of spin only magnetic moments, the susceptibility results suggest that the P centers in the oxidized protein are S = 5/2 systems.