Particles of nanometer size (nanoparticles) supported on well-characterized oxide surfaces are of particular interest to model the high complexity of real catalysts to answer questions such as the rule of intrinsic size effects and the influence of the support.(1,2) Model systems so far consisted of size-distributed nanoparticles deposited on oxide substrates,(3-5) which do not allow an unambiguous determination of the cluster＇s chemical nature. Here, we report on the size-dependent chemical reactivity of nickel clusters, size selected and deposited with low energy (0.2 eV/atom) on thin MgO(100) films; Monodispersed Ni-30 clusters show a higher reactivity for CO dissociation than Ni-11 and Ni-20. In particular, Ni-30 clusters are extremely reactive and dissociate up to 10 CO molecules at temperatures below 280 K. Our results demonstrate that such small, supported clusters are unique for catalytic reactions not only due to their high surface-to-volume ratio but essentially because of the distinctive properties of different cluster sizes.