Measurements of the core and ligand monolayer compositions of small gold nanoparticles (NPs) using electrospray ionization (ESI) mass spectrometry were performed by incorporating ionization tags, methoxy penta(ethylene glycol) thiolate ligands (-S-PEG), into the ligand monolayers via ligand exchange. During ESI, alkali metal ions (M+) coordinate to the -S-PEG ligands and give the NPs positive charge. Atomically precise, high-resolution measurements show unequivocally that the NP composition is Au-25(ligand)(18). The predominant ions, M4Au25(ligand)(18)(3+) and M5Au25(ligand)(18)(4+), have 1- charge on the core. Because ligand exchange is a statistical process, there is a distribution of mixed-monolayer exchange products, which is reflected in the mass spectra.