The decagallane(6) Ga(10)R'(6) [R' = Si(CMe(3))(3)] adopts a quite unusual structure, which might be described as being derived from a pentagonal bipyramidal core, which is threefold capped. An alternative description is that of a conjuncto-cluster. The structure of the anionic cluster [Ga(10)R'(6)](-) as well as that of Ga(10)R''(6) [R'' = Si(SiMe(3))(3)] are described as being built of fused octahedra. Thus, this family of decagallanes is unique in showing structural isomers in the cluster core, giving hints to the pathways of formation of these clusters, too. The novel cluster compound is characterized by X-ray crystallography. Isomeric decagallanes(6) and structural changes on reduction are studied by DFT methods.