The in situ formation of an iridium nanoparticles dispersion obtained from the reduction of the organometallic precursor [Ir(cod)CI](2) in I-n-butyl-3-methyl imidazolium hexafluorophosphate ionic liquid by molecular hydrogen is an autocatalytic process. These nanoparticles possess a small size and a monomodal size distribution of 2.0 +/- 0.4 nm iridium nanoparticles can be confirmed by TEM and XRD analysis before and after the catalytic process. The I-decene hydrogenation by the Ir(0) nanoparticles in the ionic liquid follows the classical monomolecular surface reaction mechanism (v = kc K[S]/I+K[S]). The reaction rate is a mass controlled process under hydrogen pressure < 4 atm. The catalytic kinetic constant (k(c)) and the adsorption constant (K) under hydrogen pressures >= 4 atm are independent of the hydrogen concentration indicating zero order dependence on hydrogen pressure and the reaction depends only of the I-decene concentration in the ionic liquid. (c) 2005 Elsevier B.V. All rights reserved.