The methylation reaction of (C,Me,)2M%( &3z)(~-S)2 (Ia) leads to two isomers of composition (C,Me,),Mo,(SMe),S, (IVa and IVb) which were separated by column chromatography. As an intermediate product of this reaction the iodinecontaining adduct (C,Me,),Mo$,I, is formed. All compounds were investigated by means of 'H and 95Mo NMR spectroscopy a s well as by an X-ray structure analysis in the case of IVa. Thus, the SCH, ligands in IV are found to be arranged in a tram position. As a further result a cleavage of the $-disulfur bridge, originally present in Ia, has been found to occur in the second step of the reaction sequence.