For catalytic metal nanoparticles (<2 nm), structural information is rarely generated using conventional X-ray diffraction (XRD) owing to the broad peaks and partial oxidation of the nanoparticles on exposure to air. Here we report how in situ synchrotron XRD provides structural information on reduced 1–2 nm Pt nanoparticles, which are unobservable by XRD when measured in air. Furthermore, for larger metal particles (>2 nm) where diffraction patterns of the metallic phase are obtainable in air, we show that on exposure to air the surface is oxidized with a metallic core producing misleading results with respect to particle size and lattice parameter. Results from XRD are cross-correlated with scanning transmission electron microscopy and three other synchrotron X-ray techniques, small angle diffraction (SAXS), pair distribution function (PDF) and X-ray absorption spectroscopy (XAS), to provide detailed characterization of the structure of very small nanoparticles in the metallic phase.