Reaction of the cluster salt [(η 5 -Cp ' ) 3 Mo 3 S 4 ][pts] ([1][pts], Cp ' =methylcyclopentadienyl; pts=p-toluenesulfonate) with CuCl yielded a new heterobimetallic cluster, [(η 5 -Cp ' ) 3 Mo 3 S 4 Cu(Cl)][pts] ([2][pts]). X-ray crystal structure determination of [1][pts] and [2][pts] showed that the incorporation of CuCl into the Mo 3 S 4 cluster core has only minor consequences on the Mo-Mo and Mo-S distances. The metal atoms in the cluster core of [2] + form an almost regular tetrahedron. The [2] + cation conforms to an idealized C 3 v symmetry ignoring the Cp ' groups since the Cu-Cl bond is almost aligned with the axis defined by the Cu-S(4) cube diagonal.