Both oxidation and sublimation are expected to be sensitive to the NP surface structure. For graphitic or other black-carbon NPs, we expect that fully-coordinated sp 2 sites should be unreactive toward O2, but under-coordinated carbon at exposed basal plane edges or defects should be reactive. Unreconstructed diamond surfaces also have many undercoordinated atoms, but various reconstructions are possible, thus the negligible reactivity observed in Fig. could be taken as evidence that nanodiamonds are inherently O2-resistant at 1500 K, or that the diamond NP had already evolved to an O2-resistant surface structure during the 1900 K sublimation period. The 1900 K sublimation rates in Fig. (0.4 Da/sec/nm 2 for graphite; 0.27 Da/sec/nm 2 for diamond) correspond to desorption of ≤ 1% of the surface atoms/second. Thus, while sublimation is still rather slow at 1900 K, and thus should be dominated by under-coordinated sites with low Cn desorption energies, 1500 K oxidation is clearly far more sensitive to surface structure, and therefore is a superior probe for the presence of reactive sites on NP surfaces.