Cytochromes c 3 are tetrahaem electron transfer proteins isolated from Desulfovibrio spp. which play an important role in the metabolism of sulfur compounds and hydrogen [1]. They have a molecular weight of approx. 13,000 Daltons [1, 2]. The four haems, mesoporphyrins, are covalently bound to the polypeptide chain through thioether linkages provided by cysteinyl residues. The axial ligands are two hystidinyl residues [3, 4] and all the haems have negative, although different mid-point redox potentials [5–8]. Several physico-chemical techniques, mainly Mössbauer [9], EPR [4–7], NMR [3, 10–16], cyclic voltammetry, and different pulse polarography [6, 7], have been applied to elucidate the mechanism of electron transfer in cytochrome c 3 . However, the mechanism by which electrons are transferred between cytochrome c 3 and other electron carriers is only poorly understood. NMR has been shown to be a suitable technique to elucidate the electron exchange mechanism of cytochrome