Ultrasmall nanoparticles of platinum group metal oxides (core diameter of about 1.8 nm) were prepared by alkaline hydrolysis of metal precursors in the presence of NaBH4 and by colloidal stabilization with tripeptide glutathione. We obtained water-dispersed nanoparticles of Rh2O3, PdO, RuO2, IrO2, Os/OsO2, and Pt/PtO. Their size was probed using high-resolution transmission electron microscopy, differential centrifugal sedimentation, small-angle X-ray scattering, and diffusion-ordered 1H NMR spectroscopy (1H DOSY). Their oxidation state was clearly determined using X-ray photoelectron spectroscopy, X-ray powder diffraction, and electron diffraction. The chemical composition of the nanoparticles, that is, the ratio of the metal oxide core and glutathione capping agent, was quantitatively determined by a combination of these methods.