[Ir-6(CO)(16)] was formed in the pores of zeolite NaY by adsorption of [Ir(CO)(2)(acac)] followed by treatment in CO+H-2. [Ir-6(CO)(15)](2-) in zeolite NaX was prepared similarly. Each sample was characterized by high-resolution transmission electron microscopy. The images indicate the presence of the iridium clusters in the zeolite micropores, with almost no scattering centers indicating iridium outside these pores. The supported [Ir-6(CO)(16)] and [Ir-6(CO)(15)](2-), which have previously been characterized by infrared and extended X-ray absorption fine structure spectroscopies, are among the most uniform and structurally best defined supported metal clusters.