A current goal in heterogeneous catalysis is to transfer the synthetic, as well as developing mechanistic, insights from the modern revolution in nanoparticle science to the synthesis of supported-nanoparticle heterogeneous catalysts. In a recent study (Mondloch, J. E.; Wang, Q; Frenkel, A. I.; Finke, R. G. J. Am. Chem. Soc. 2010, 132, 9701-9714), we initialized tests of the global hypothesis that quantitative kinetic and mechanistic studies, of supported-nanoparticle heterogeneous catalyst formation in contact with solution, can provide synthetic and mechanistic insights that can eventually drive improved syntheses of composition-, size-, and possibly shape-controlled catalysts. That study relied on the development of a well-characterized Ir(1,5-COD)Cl/gamma-Al2O3 precatalyst, which, when in contact with solution and H-2, turns into a nonaggregated Ir(0)(similar to 900)/gamma-Al2O3 supported-nanoparticle heterogeneous catalyst. The kinetics of the Ir(1,5-COD)Cl/gamma-Al2O3 to Ir(0)(similar to 900)/gamma-Al2O3 conversion were followed and fit by a two-step mechanism consisting of nucleation (A -> B, rate constant k(1)) followed by autocatalytic surface growth (A + B -> 2B, rate constant k(2)). However, a crucial, but previously unanswered question is whether the nucleation and growth steps occur primarily in solution, on the support, or possibly in both phases for one or more of the catalyst-formation steps. The present work investigates this central question for the prototype Ir(1,5-COD)Cl/gamma-Al2O3 to Ir(0)(similar to 900)/gamma-Al2O3 system. Solvent variation-,gamma-Al2O3-, and acetone-dependent kinetic data, along with UV-vis spectroscopic and gas-liquid-chromatography (GLC) data, are consistent with and strongly supportive of a supported-nanopartide formation mechanism consisting of Ir(1,5-COD)Cl (solvent) dissociation from the gamma-Al2O3 support (i.e., from Ir(1,5-COD)Cl/gamma-Al2O3), solution-based nucleation from that dissociated Ir(1,5-COD)Cl(solvent) species, fast Ir(0) nanopartide capture by gamma-Al2O3, and then subsequent solid-oxide-based nanoparticle growth from Ir(0)(n)/gamma-Al2O3 and with Ir(1,5-COD)Cl(solvent), the first kinetically documented mechanism of this type. Those data disprove a solid-oxide-based nucleation and growth pathway involving only Ir(1,5-COD)Cl/gamma-Al2O3 and also disprove a solution-based nanoparticle growth pathway involving Ir(1,5-COD)Cl(solvent) and Ir(0)(n) in solution. The present mechanistic studies allow comparisons of the Ir(1,5-COD)Cl/gamma-Al2O3 to Ir(0)(similar to 900)/gamma-Al2O3 supported-nanoparticle formation system to the kinetically and mechanistically well-studied, Ir(1,5-COD)center dot P2W15Nb3O628- to Ir(0)(similar to 300)center dot(P2W15Nb3O628-)(n)(-8n) solution-based, polyoxoanion-stabilized nanopartide formation and stabilization system. That comparison reveals closely analogous, solution Ir(1,5-COD) or Ir(1,5-COD)Cl-mediated, mechanisms of nanoparticle formation. Overall, the hypothesis supported by this work is that these and analogous studies hold proise of providing a way to transfer the synthetic and mechanistic insights, from the modern revolution in nanoparticle synthesis and characterization in solution, to the rational, mechanism-directed syntheses of solid oxide-supported nanoparticle heterogeneous catalysts, also in contact with solution.