It could be an unintentional effect to deposit metal nanoparticles on a simple Teflon-coated magnetic stirring bar. Rhodium nanoparticles, as an example, were reproducibly deposited onto a standard, commercial Teflon-coated magnetic stirring bar by easy and rapid microwave-assisted decomposition of the metal carbonyl precursor Rh-6(CO)(16) in the ionic liquid 1-n-butyl-3-methyl-imidazolium tetrafluoroborate. Such metal nanoparticle deposits are not easy to remove from the Teflon surface by simple washing procedures and present active catalysts which one is not necessarily aware of. Barely visible metal-nanoparticle deposits on a stirring bar can act as trace metal impurities in catalytic reactions. As a proof-of-principle the rhodium-nanoparticle deposits of 32 mu g or less Rh metal on a 20 mm x 6 mm magnetic stirring bar were shown to catalyze the hydrogenation reaction of neat cyclohexene or benzene to cyclohexane with quantitative conversion. Rhodium-nanoparticle-coated stirring bars were easily handable, separable and re-usable catalyst system for the heterogeneous hydrogenation with quantitative conversion and very high turnover frequencies of up to 32,800 mol cyclohexene x (mol Rh)(-1) x h(-1) under organic-solvent-free conditions. (C) 2012 Elsevier B.V. All rights reserved.