Nearly monodispersed 30 +/- 4 Angstrom Ir-similar to 900 nanoclusters have been prepared by hydrogen reduction of a polyoxoanion-supported Ir(I) complex, (Bu(4)N)(5)Na-3[(1,5-COD)Ir.P2W15Nb3O62] (1,5-COD is 1,5-cyclooctadiene), in acetone solution. The Ir-similar to 900 nanoclusters can be isolated as a black powder and redissolved in acetone or CH3CN to give astable, amber solution. Extensive characterizations of the Ir-similar to 900 nanoclusters have been made by means of TEM, electron diffraction, electrophoresis, ultracentrifugation solution molecular-weight measurements, fast-atom bombardment mass spectroscopy, elemental analysis, and IR and UV-vis spectroscopy; their average chemical composition is [Ir(0)(similar to 900) (P4W30Nb6O12316-)(similar to 60)](BU4N)(similar to 660) Na-similar to 300, in which the polyoxoanion is found to be in its oxidized and Nb-O-Nb bridged aggregate form, P4W30Nb6O12316-. Electron diffraction studies show that the nanoclusters consist of cubic close-packed (ccp) Ir metal cores; electrophoresis and other techniques establish that the nanoclusters are stabilized in solution by the adsorption of the polyoxoanions on their outer surfaces. Smaller, ca. 20-Angstrom Ir-similar to 300 nanoclusters have also been reproducibly prepared and characterized from (Bu(4)N)(5)Na-3[(1,5-COD)Ir.P2W15Nb3O62] and from (Bu(4)N)4-Na-2[(1,5-COD)Ir.SiW9Nb3O40] during the catalytic hydrogenation of cyclohexene; the average chemical composition of the Ir-similar to 300 nanoclusters isolated from (Bu(4)N)(5)Na-3[(1,5-COD)Ir.P2W15Nb3O62] is [Ir(0)(similar to 300)(P(4)W(30)Nb6-O-123(16-))(similar to 33)](BU4N)(similar to 300)Na-similar to 233. The Ir-similar to 300 nanoclusters are unusual in their unexpected +/-10% kinetic reproducibility in catalysis (cyclohexene hydrogenation) and also in their stability including their isolability. In addition, the Ir-similar to 300 nanoclusters catalyze the reduction of polyoxoanions to give intense blue, W-v-containing, "heteropolyblue" species. Evidence supporting a homogeneous nucleation, then autocatalytic growth mechanism for the formation of the nearly monodispersed Ir nanoclusters is also presented and discussed. The autocatalysis is the key to the separation in time of the nanocluster nucleation and growth steps; this separation is, in turn, an essential component for the formation of monodispersed nanoclusters (i.e., in the absence of micellar or other nanocluster growth-inhibiting microstructures).