In recent work we showed that Ziegler-type nanoparticles made from [Ir(1,5-COD)(mu-O2C8H15)](2) plus AlEt3 are an unusually thermally stable (>= 30 min at 200 degrees C), hydrocarbon-solvent soluble, high catalytic activity nanoparticle catalyst (I.K. Hamdemir, S. Ozkar, K.-H. Yih, J.E. Mondloch, R.G. Finke, ACS Catal. 2 (2012) 632-641). As such, they are analogous to-and currently the cleanest and best characterized model system for-Ziegler-type nanoparticles made from Co or Ni precatalysts plus AlEt3 which are used industrially to hydrogenate similar to 1.7 x 10(5) metric tons of styrenic block copolymers per year (for a review of the area, see W.M. Alley, I.K. Hamdemir, K.A. Johnson, R.G. Finke, J. Mol. Catal.: A Chem. 315 (2010) 1-27). The key question addressed in the present paper is "What is the nature of the AlEt3-derived stabilizer species?" for the unusually stable and active Ziegler-nanoparticles formed from [Ir(1,5-COD)(mu-O2C8H15)](2) plus AlEt3. Specifically tested herein are four primary hypotheses for the AlEt3-derived stabilizer(s) in the Ir(0)(n) Ziegler-nanoparticle system: (i) that the key stabilizer is neutral (i.e., uncharged) aluminum alkyl carboxylates following precedent from the work of Shmidt and Bonnemann; (ii) that the key stabilizer is anionic [AlEt3(O2C8H15)](-); (iii) that a key stabilizer is the AlEt3 (or its derivatives) reacting with the Ir(0)(n) nanoparticle surface; or (iv) that an important AlEt3-derived stabilizer is Al-O-Al containing alkylalumoxanes formed from any water present. The results obtained rule out (ii), but provide strong evidence for (iii), as well as evidence consistent with (i) and (iv), as stabilizers in Ziegler-nanoparticles. A pictorial scheme (Scheme 2) is provided as a working hypothesis for the stabilization mode(s) of Ziegler-nanoparticles and as a way to focus and expedite the needed additional composition and structural studies of Ziegler-nanoparticle stabilizers. (c) 2013 Elsevier B.V. All rights reserved.