The gas phase fragmentation reactions of sulfur-rich [Mo3S7Br6]2− (12−), [Mo3S7(bdt)3]2− (22−), and [Mo3S4(bdt)3]2− (32−) (bdt = benzenedithiolate) complexes have been investigated by electrospray ionization (ESI) tandem mass spectrometry and theoretical calculations at the density functional theory level. Upon collision induced dissociation (CID) conditions, the brominated 12− dianion dissociates through two sequential steps that involves a heterolytic Mo−Br cleavage to give [Mo3S7Br5]− plus Br− followed by a two-electron redox process that affords [Mo3S5Br5]− and diatomic S2 sulfur. Dianion [Mo3S7(bdt)3]2− (22−) dissociates through two sequential redox processes evolving diatomic S2 sulfur and neutral bdt to yield [Mo3S5(bdt)3]2− and [Mo3S5(bdt)2]2−, respectively. Conversely, dianion [Mo3S4(bdt)3]2− (32−), with sulfide instead of disulfide S22− bridged ligands, remains intact under identical fragmentation conditions, thus highlighting the importance of disulfide ligands (S22−) as electron reservoirs to trigger redox reactions. Regioselective incorporation of 34S and Se at the equatorial position of the Mo3S7 cluster core in 12− and 22− have been used to identify the product ions along the fragmentation pathways. Reaction mechanisms for the gas-phase dissociation pathways have been elucidated by means of B3LYP calculations, and a comparison with the solution reactivity of Mo3S7 and Mo3S4 clusters as well as closely related Mo/S/dithiolene systems is also discussed.