Kinetic studies, using stopped-flow spectrophotometry, on the reactions of [M-4(SPh)(10)](2-) (M = Fe or Co) with PhS- to form [M(SPh)(4)](2-) are described, as are the reactions between [M-4(SPh)(10)](2-) and [MoS4](2-) to form [S2MoS2Fe(SPh)(2)](2-) or [S2MoS2CoS2MoS2](2-). The kinetics of the reactions with PhS- are consistent with an initial associative substitution mechanism involving attack of PhS- at one of the tetrahedral M sites of [M-4(SPh)(10)](2-) to form [M-4(SPh)(11)](3-). Subsequent or concomitant cleavage of a mu-SPh ligand, at the same M, initiates a cascade of rapid reactions which result ultimately in the complete rupture of the cluster and formation of [M(SPh)(4)](2-). The kinetics of the reaction between [M-4(SPh)(10)](2-) and [MoS4](2-) indicate an initial dissociative substitution mechanism at low concentrations of [MoS4](2-), in which rate-limiting dissociation of a terminal thiolate from [M-4(SPh)(10)](2-) produces [M-4(SPh)(9)](-) and the coordinatively unsaturated M site is rapidly attacked by a sulfido group of [MoS4](2-). It is proposed that subsequent chelation of the MoS4 ligand results in cleavage of an M-mu-SPh bond, initiating a cascade of reactions which lead to the ultimate break-up of the cluster and formation of the products, [S2MoS2Fe(SPh)(2)](2-) or [S2MoS2CoS2MoS2](2-). With [Co-4(SPh)(10)](2-), at higher concentrations of [MoS4](2-), a further substitution pathway is evident which exhibits a second order dependence on the concentration of [MoS4](2-). The mechanistic picture of cluster disruption which emerges from these studies rationalizes the "all or nothing" reactivity of [M-4(SPh)(10)](2-).