The reaction of the incomplete-cuboidal [W₃Se₄(OH)₃(dmpe)₃]⁺ ([1]⁺) cluster with acetic acid in acetonitrile solution leads to cluster fragmentation with formation of the dinuclear [W₂Se₂(µ-Se)₂(µ-CH₃CO₂)(dmpe)₂]⁺ ([2]⁺) complex. The X-ray structure of [2]PF₆ presents two equivalent metal centres bridged by one acetate ligand. Each W atom is additionally coordinated by one terminal selenium atom, two bridging selenido and two diphosphane phosphorus atoms in an essentially octahedral environment. Stopped-flow and conventional UV-vis studies indicate that fragmentation of [1]⁺ into [2]⁺ occurs through a complex mechanism. Three steps can be distinguished in the stopped-flow time scale, all of them showing a first order dependence with respect to the acetic acid concentration, followed by very slow spectral changes that lead to the formation of [2]⁺. Phosphorus NMR, electrospray ionization mass spectrometry (ESI-MS) and tandem mass spectrometry (ESI-MS/MS) have been used to identify the nature of the reaction intermediates formed in the different steps. These studies indicate that the first two steps correspond to the formal substitutions of the hydroxo ligands at two metal centres by terminal acetate ligands. The third step involves bridging of one of the terminal acetate ligands, which actually prepares the trinuclear cluster to afford the acetate-bridged [W₂Se₂(µ-Se)₂(µ-CH₃CO₂)(dmpe)₂]⁺ ([2]⁺) complex. Although the precise details of the final conversion to [2]⁺ have not been established, the results obtained by combination of the different experimental techniques provide a complete picture of the speciation of the cluster [1]⁺ in acetonitrile solutions containing acetic acid.