Abstract: The kinetics and mechanisms of the substitution reactions of (μ,μ-SCH_2S)Fe_2 (CO)_6 with PPh_3, PMe_2Ph, PEt_3, P(n-Bu)_3 and (μ-PhCH_2S)(μ-RS)Fe_2(CO)_6(R=Me, Et) with PPh_3 in various solvents have been studied by means of UV-spectra and ~1H NMR. The results support the mechanism of Ia (associative interchange) for the substitution reactions of first step and Id (dissociative interchange) for the reactions of second step. The electronic effects of the ligands, steric effects of the ligands and the parent complexes and the solvent effecs on the reaction rates have been discussed. It shows that the reaction rate is presumably determined by the volume of the incoming ligands. the nucleophilic solvents such as DMF can promote the reaction rate by means of base catalysis.