The reactions of Rh4(CO)12 with phosphine and arsine ligands have been investigated. Treatment of Rh4(CO)12 with one or two moles of ligand under mild conditions yields the compounds Rh4(CO)11L and Rh4(CO)10L2 respectively, where L = PPh3, P(p-CH3C6H4)3, P(p-FC6H4)3, and AsPh3, 2L = Ph2PCH2CH2PPh2, in which the basic structure of the parent cluster carbonyl is thought to be retained. Further reactions of the disubstituted derivatives with carbon monoxide under pressure, preferably in the presence of excess phosphine, yield the unstable dinuclear species Rh2(CO)6L2, where L = PPh3, P(p-CH3C6H4)3, and P(p-FC6H4)3, which readily revert to the tetranuclear compounds. The significance of this dimer-tetramer equilibrium is discussed in relation to the difficulty experienced in characterising Rh2(CO)8. Infrared spectral evidence for the reversible formation of Rh2(CO)8 from the reaction of Rh4(CO)12 with high pressures of carbon monoxide at low temperatures is also described.