Organically capped metal nanoparticles are an attractive alternative to more conventional oxide-passivated materials, due to the lower reaction temperatures and the possibility of tuning the organic coating. Sonochemical methods have been used to produce small (∼5 nm average size) air-stable aluminum nanoparticles capped with oleic acid. In order to understand the nature of the metal−organic bonding in the nanoparticles, we have used FTIR, XPS, and TOFMS−TPD techniques to study the organic passivation layer and its desorption at elevated temperatures. In the present case we find that the organic layer appears to be attached via Al−O−C bonds with the C atom formerly involved in the carboxylic acid functional group.