The reduction of ferricytochrome c ( cyt c^III), as well as some iron ligand complexes and chemically modified derivatives of cyt c^III, by two molybdenum(V)-cysteine complexes has been investigated as a model for electron transfer in molybdenum enzymes. The reduction of cyt c^III by di-μ-oxo-bis[oxo(L-cysteinato)molybdate(V)] (I) is first order in cyt c^III and zero order in I over a wide range in concentration of I at neutral pH. The reduction of cyt c^III by μ-oxo-bisfoxodihydroxo(L-cysteinato) molybdate(V)] (II) is first order in both reactants and has a rate several orders of magnitude greater than the rate of reduction by I. Activation parameters have been determined for both reactions. The reduction of two pyridoxal phosphate derivatives of cyt c^III by I proceeds at the same rate as the reduction of native cyt c^III, while the mononitromonotyrosyl derivative is reduced somewhat faster. The reduction of the N-formyltryptophyl derivative of cyt c^III by I is biphasic, with the fast phase proceeding with a rate similar to the native cyt c^III and the slow phase being somewhat slower. The reduction by I of all derivatives studied is first order in cyt c^III and zero order in I, suggesting the mechanism does not change with these chemically modified forms of cyt c^III. The pyridoxal phosphate and mononitromonotyrosyl derivatives of cyt c^III are reduced by II with rates similar to the native cyt c^III and by the same mechanism. The reduction of the N-formyltryptophyl derivative by II has complicated kinetics which suggest the presence of at least two formyl-cyt c^III species. The reduction of the cyanide, azide and imidazole complexes of cyt c^III by II occurs at a greatly reduced rate and by a different mechanism than the reduction of native cyt c^III The rate of reduction of the cyanide and azide complexes is dependent on the rate of dissociation of the ligand cyt c^III complex and independent of the concentration of II. The reduction of the imidazole complex by II is much more complicated, with the rate independent of II at very low concentrations but shows a dependence on the concentration of II at higher concentrations. Rate constants have been determined for the reactions and possible mechanisms have been developed. The results have been discussed with regard to electron transfer in molybdenum enzymes and electron transfer to cyt c^III This study is especially valuable as a model for the hepatic sulfite oxidase enzyme.