Under anaerobic conditions the molybdenum-iron protein (MoFe protein) from Azotobacter vinelandii can be reversibly oxidized with thionine. Electron paramagnetic resonance studies reveal that the oxidation proceeds in two distinct phases: the MoFe protein can be oxidized by four electrons without loss of the EPR signal from the S = 3/2 cofactor centers. A second oxidation step, involving two electrons, leads to the disappearance of the cofactor EPR signal. In order to correlate the events during the thionine titration with redox reactions involving individual iron centers we have studied the MoFe proteins from A vinelandii and Clostridium pasteurianum with Mössbauer spectroscopy. Spectra were taken in the temperature range from 1.5 K to 200 K in applied magnetic fields of up to 54 kG. Analysis of the Mössbauer data allows us to draw three major conclusions: (1) the holoprotein contains 30 +/- 2 iron atoms. (2) Most probably, 12 iron atoms belong to two, apparently identical, iron clusters (labeled M) which we have shown previously to be structural components of the iron and molybdenum containing cofactor of nitrogenase. The M-centers can be stabilized in three distinct oxidation states, MOXe- in equilibrium MNe- in equilibrium MR. The diamagnetic (S = 0) state MOX is attained by oxidation of the native state MN with either thionine or oxygen. MR is observed under nitrogen fixing conditions. (3) The data strongly suggest that 16 iron atoms are associated with four iron centers which we propose to call P-clusters. Each P-cluster contains four spin-coupled iron atoms. In the native protein the P-clusters are in the diamagnetic state PN, yielding the Mössbauer signature which we have labeled previously 'components D and Fe2+'. Three irons of the D-type and one iron of the Fe2+-type appear to comprise a P-cluster. A one-electron oxidation yields the paramagnetic state POX. Although the state POX is characterized by half-integral electronic spin a peculiar combination of zero-field splitting parameters and spin relaxation renders this state EPR-silent. Spectroscopically, the P-clusters are novel structures; there is, however, evidence that they are closely related to familiar 4Fe-4S centers.