In situ X-ray photoelectron spectroscopy analysis of Ir(0) nanoparticles (1.6 +/- 0.3 nm) dispersed in imidazolium ionic liquid (EMI,EtSO4) shows evidences of the effective interaction between the metallic clusters and the surrounding liquid. By monitoring the C 1s signal of the ionic liquid one observes a change of the binding energy in one of its components (C2) when in the presence of Ir nanoparticles. This result was corroborated by isotope labeling experiments. (C) 2009 Elsevier B.V. All rights reserved.