Reaction of [(C4H9)(4)N](2)[Fe4S4(SR)(4)] (R = C6H5, C2H5) with (CH3O)(3)PO in DMSO-d(6) afforded [(C4H9)N](2){Fe4S4(SR)(3)[(CH3O)(2)PO2]} and CH3SR as revealed by H-1 and P-31{H-1} NMR spectroscopy. The more reduced species [(C2H5)(4)N](3)[Fe4S4(SC2H5)(4)] gave uncoordinated (CH3O)(2)PO2- and CH3SC2H5 in addition to an unidentified iron thiolate species. Stoichiometric methylation of mononuclear [(C2H5)(4)N](2)[Fe(SC2H5)(4)] by (CH3O)(3)PO afforded [Fe-2(SC2H5)(6)](2-) as well as free (CH3O)(2)PO2- and CH3SC2H5 Kinetic studies revealed the rate constant for methylation of [(C2H5)(4)N](3)[Fe4S4(SC2H5)(4)] to be more than 200-fold higher than that of the oxidized analogues [(C4H9)(4)N](2)[Fe4S4(SR)(4)] (R = C6H5, C2H5). The compound [(C2H5)(4)N](2)[Fe(SC2H5)(4)] had the highest rate constant, greater than or equal to 5 x 10(-3) s(-1) at concentrations of 5.0 mM in complex and 1.0 mM in (CH3O)(3)PO. Attempts to prepare site-differentiated tetranuclear iron-sulfur complexes by removing one thiolate via methylation and addition of second, capping ligands are described. These results are discussed in the context of protein metal thiolate moieties that transfer methyl cations for substrate synthesis, such as carbon monoxide dehydrogenase/acetyl coenzyme A synthase, and repair of DNA alkylation damage.