NMR studies of CO adsorbed on small Pt particles show evidence of changes in the metallic nature of these particles with size. Large particles on silica or the exterior of zeolite crystallites have conduction-band electrons that cause a Knight shift for adsorbed CO. Small particles in zeolite cavities are diamagnetic clusters, and yield spectra for linear and bridging carbonyls similar to those of transition-metal cluster compounds. C-13 NMR of CO offers a simple probe of metal dispersion and particle size for these Pt catalysts and other noble metal systems.