Supported metal particles may exhibit properties fundamentally different from the corresponding bulk materials. To gain insight into principles underlying size- and structure-dependent phenomena, a structural characterization of small aggregates at the atomic level is crucial, while far from straightforward. A long-standing question in the study of supported clusters and metal-oxide interfaces concerns the extent of metal-oxide charge transfer. We show that infrared spectroscopy, utilizing carbon monoxide as a probe molecule, may provide valuable information on both structure and charge of ultrasmall metal aggregates and single metal atoms. To create supported particles containing only few atoms or even a single atom, submonolayer amounts of the transition metals palladium, rhodium, and iridium were vapor-deposited onto a thin, well-ordered alumina film at low substrate temperatures. Scanning tunneling microscopy served to characterize nucleation behavior and average particle size. Sharp, discrete features in the infrared spectra of adsorbed CO are due to uniform metal (M) carbonyls, most notably the mono- and dicarbonyl species MCO and M(CO)(2). The thermal behavior of such carbonyls is reflected in the thermal evolution of their infrared signatures. Comparing the C-O stretching frequencies of MCO species on the aluminum oxide film to those of their matrix-isolated neutral and charged counterparts, the charge of the metal centers is estimated. In this way, the extent of metal-oxide charge transfer at point defects and regular sites of the alumina film is shown to be smaller than +/-0.2 elementary charges. By contrast, Rh atoms more strongly bound to oxide line defects are oxidized by the alumina substrate.