Ziegler-type hydrogenation catalysts, those made from a group 8-10 transition metal precatalyst and an AIR(3) cocatalyst, are often used for large scale industrial polymer hydrogenation; note that Ziegler-type hydrogenation catalysts are not the same as Ziegler-Natta polymerization catalysts. A review of prior studies of Ziegler-type hydrogenation catalysts (Alley et al. J. Mot CataL A: Chem. 2010, 315, 1-27) reveals that a 50 year old problem is identifying the metal species present before, during, and after Ziegler-type hydrogenation catalysis, and which species are the kinetically best, fastest catalysts that is, which species are the true hydrogenation catalysts. Also of significant interest is whether what we have termed "Ziegler nanoclusters" are present and what their relative catalytic activity is. Reported herein is the characterization of an Ir Ziegler-type hydrogenation catalyst, a valuable model (vide infra) for the Co-based industrial Ziegler-type hydrogenation catalyst, made from the crystallographically characterized [(1,5-COD)Ir(mu-O2C8H15)](2) Precatalyst plus AlEt3. Characterization of this Ir model system is accomplished before and after catalysis using a battery of physical methods including Z-contrast scanning transmission electron microscopy (STEM), high resolution (HR)TEM, and X-ray absorption fine structure (XAFS) spectroscopy. Kinetic studies plus Hg(0) poisoning experiments are then employed to probe which species are the fastest catalysts. The main findings herein are that (i) a combination of the catalyst precursors [(1,5-COD)Ir(mu-O2C8H15)](2) and AlEt3 gives catalytically active solutions containing a broad distribution of Irn species ranging from monometallic Ir complexes to nanometer scale, noncrystalline Ir-n nanoclusters (up to Ir-similar to 100 by Z-contrast STEM) with the estimated mean Ir species being 0.5-0.7 nm, Ir similar to 4-15 clusters considering the similar, but not identical results from the different analytical methods; furthermore, (ii) the mean Ir-n species are practically the same regardless of the AI/Ir ratio employed, suggesting that the observed changes in catalytic activity at different AI/Ir ratios are primarily the result of changes in the form or function of the Al-derived component (and not due to significant AlEt3-induced changes in initial Ir-n nuclearity). However (iii), during hydrogenation, a shift in the population of Ir species toward roughly 1.0-1.6 nm, fcc Ir(0)(similar to 40-150), Ziegler nanoclusters occurs with, significantly, (iv) a concomitant increase in catalytic activity. Importantly, and although catalysis by discrete subnanometer Ir species is not ruled out by this study, (v) the increases in activity with increased nanocluster size, plus Hg(0) poisoning studies, provide the best evidence to date that the approximately 1.0-1.6 nm, fcc Ir(O)(similar to 40-150), heterogeneous Ziegler nanoclusters are the fastest catalysts in this industrially related catalytic hydrogenation system (and in the simplest, Ockham's Razor interpretation of the data). In addition, (vi) Ziegler nanoclusters are confirmed to be an unusual, hydrocarbon-soluble, highly coordinatively unsaturated, Lewis-acid containing, and highly catalytically active type of nanocluster for use in other catalytic applications and other areas.