Au° atoms in hydrophobized gold nanoparticles were oxidized to Au³⁺ ions, nearly quantitatively, simply by treatment with N-bromosuccinimide and sonication. ¹H NMR results indicate that the octadecylamine molecules are detached from the Au° surface by NBS. The bromine molecules released by NBS are suggested to be the species responsible for the oxidation of Au° to Au³⁺, which is supported by the observation that addition of molecular bromine also leads to similar results. A gratifying feature is that the Au³⁺ ions could be subsequently reduced back to Au nanoparticles.