The formation of iridium clusters from supported mononuclear iridium complexes in H-2 at 300 K and 1 bar was investigated by spectroscopy and atomic-resolution scanning transmission electron microscopy. The first steps of duster formation from zeolite-supported Ir(C2H4)(2) complexes are triggered by the activation of H-2 and the formation of iridium hydride, accompanied by the breaking of iridium-support bonds. This reactivity can be controlled by the choice of ligands on the iridium, which include the support.