Silica supported iridium and rhodium nanoparticles were prepared via surface organometallic chemistry (SOMC). Following the synthesis and characterization of organometallic molecular precursors [(COD)MOSiOR](2) (M = Ir or Rh; R = Me or Si(OtBu)(3)), their controlled grafting on partially dehydroxylated silica yields isolated dinuclear surface species, as determined by H-1 and C-13 solid state NMR as well as infrared transmission spectroscopies and elemental analysis. The decomposition under hydrogen of the well-defined surface species affords a narrow size distribution and a homogeneous spatial repartition of small M(0) particles despite a high metal density (1.2 +/- 0.3 nm and 1.4 +/- 0.3 nm for Ir and Rh, respectively). A combination of transmission electronic microscopy and gas chemisorption provides the H-2 and CO adsorption stoichiometries on the metal surface, which are highly dependent on the precursor and the preparation route, indicating the necessity to control each step and the danger to determine particle size solely from chemisorption studies for small iridium and rhodium supported particles. (C) 2014 Elsevier Inc. All rights reserved.