The compounds [Ru(η⁶-C₈H₁₀)(cod)](C₈H₁₀= cycle-octa-1,3,5-triene, cod = cycle-octa-1,5-diene) and [Rh(CO)₂(η-C₅Me₅)] react in toluene at 60 °C to give the cluster compounds [RuRh₃(µ₃-CO)₂(CO)₃(η-C₅Me₅)₃](1) and [RuRh₂(µ-CO)(µ₃-CO)(CO)₂(η⁴-C₈H₁₀)(η-C₅Me₅)₂](2). The structures of these compounds were established by single-crystal X-ray diffraction studies. Compound (1) has a distorted tetrahedral RuRh₃ metal atom core [Rh–Rh 2.684(1)–2.739(1), Ru–Rh 2.620(1)–2.715(1)Å]. The rhodium atoms are each ligated by an η-C₅Me₅ group, the ruthenium atom is terminally bonded to three carbonyl groups, while the remaining two CO ligands triply and asymmetrically bridge the Rh₃ face and one RuRh₂ face of the metal polyhedron. In compound (2) the C₈H₁₀ ligand attached to the ruthenium is bicyclo[4.2.0]octa-2,4-diene. The molecule comprises a metal atom triangle [Ru–Rh 2.766(1) and 2.815(1), Rh–Rh 2.672(1)Å] and an η-C₅Me₅ group is co-ordinated to each rhodium atom. The four CO ligands show a remarkable variety of bonding modes. One carbonyl group attached to the ruthenium atom is essentially terminally bound to that centre [Ru–C–O 167.5(4)°], while a second carbonyl group attached to the ruthenium semi-bridges an edge of the metal triangle [Ru–C–O 158.1(4)°; Ru–CO 1.930(4), Rh ⋯ CO 2.412(5)Å]. The Rh–Rh vector is symmetrically bridged by another CO ligand [Rh–C–O 137.3(3) and 138.2(3)°, Rh–CO 1.989(3) and 2.000(4)Å]. The remaining carbonyl group asymmetrically bridges the face of the triangle [Ru–CO 2.247(4), Rh–CO 2.038(4) and 2.069(4)Å]. Spectroscopic studies (i.r. and n.m.r.) on (1) and (2) are reported. Both compounds show site exchange of CO groups in solution (¹³C-{^lH} n.m.r.) but the process in (1) is limited to the Ru(CO)₃ group, with the dynamic behaviour persisting at –80 °C. With (2), at room temperature, all four CO ligands exchange, but at –80 °C separate signals for the µ-CO and µ₃-CO groups are seen, although only one resonance is observed for the two CO groups attached to the ruthenium atom. These and other data are discussed, and a mechanism proposed to account for the formation of (1) and (2).