The hydroformylation of 1-alkenes can be performed in solventless conditions using ligand-modified or unmodified Rh(0) nanoparticles prepared in imidazolium ionic liquids as catalyst precursors. There is a strong influence of the nanoparticle size on the hydroformylation reaction. Aldehydes are generated when 5.0 nm Rh(0) nanoparticles are used in the hydroformylation of 1-alkenes and l/b selectivities up to 25 can be achieved by addition of Xantphos (9,9-dimethyl-4,5-bis(diphenylphosphino)xanthene). Although small nanoparticles also generate catalytically active species the chemoselectivity decreases (around 11-17% of olefin isomerization) compared to those performed with the 5.0 nm nanoparticles. In contrast, the large sized nanoparticles (15 nm) produce only small. amounts of aldehydes similar to that observed with a classical heterogeneous Rh/C catalyst precursor. TEM, XRD, IR and NMR experiments indicated that these Rh(0) nanoparticles are probably degraded under the reaction conditions into soluble mononuclear Rh-carbonyl catalytically active species. (c) 2006 Elsevier B.V. All rights reserved.