It was found that the red isomer of [Ir-6(CO)(16)] With four face-bridging carbonyl ligands isomerizes easily in solution, affording a mixture of this red isomer and of the black isomer which has four edge-bridging carbonyl ligands. Heating the solution at 80 degrees C under N-2 or air leads to [Ir-4(CO)(12)]. This behavior was unexpected because, up to now, it was believed that the [Ir-6(CO)(16)] isomers do not interconvert and because metal carbonyl clusters usually increase their nuclearity by thermal treatment.