In situ labelling and spectroscopic experiments are used to explain the key points in the stabilisation of ruthenium nanoparticles (RuNPs) generated in imidazolium-based ionic liquids (ILs) by decomposition of (eta(4)-1,5-cyclooctadiene)(eta(6)-1,3,5-cyclooctatriene)ruthenium(0), Ru(COD)(COT), under dihydrogen. These are found to be: (1) the presence of hydrides at the RuNP surface and, (2) the confinement of RuNPs in the non-polar domains of the structured IL, induced by the rigid 3-D organisation. These results lead to a novel stabilisation model for NPs in ionic liquids.