We have studied the MoFe protein from Azotobacter vinelandii OP with Mössbauer spectroscopy in applied magnetic fields up to 50 kG. The results are as follows. (1) The Mössbauer spectra of the S = 3/2 centers, which reside on the cofactor of nitrogenase, have been decomposed into six subcomponents. This suggests that each center contains 5-7, most probably 6, Fe atoms, thus confirming our earlier conclusions which were based on the quantitation of EPR data and on the assumption that the MoFe protein contains (30 +/- 2) Fe atoms. (2) Analysis of the high-field data shows that three subsites are characterized by a positive magnetic hyperfine coupling constant, A0, while A0 is negative for the other three sites. This observation demonstrates that the S = 3/2 centers are spin-coupled structures. (3) The zero-field splitting parameter D = +(6 +/- 1.5) cm-1 obtained from the Mössbauer data is in good agreement with our earlier EPR results, D approximately +5.5 cm-1. (4) The resolution of the Mössbauer spectra of the MoFe protein can be dramatically increased by employing Fourier transform deconvolution techniques. This allows a clear demonstration of spectral component S.