Figs. and show that diamond NPs are quite reactive at 1200 K, and that reactivity decreases irreversibly under both inert and oxidative conditions at higher TNP. Fig. shows an experiment to test how a diamond NP evolves under inert conditions, using O2 oxidation as a probe of changes in the number of exposed reactive sites. An NP of initial mass ~46.5 MDa was trapped and Q-stepped (purple background), carefully keeping TNP below 1150 K to minimize changes to the NP. TNP was then set to 1200 K, and the mass loss rates were measured in both argon and with 4.3 x 10 -5 Torr of O2 added (cyan background) to characterize the initial reactivity. After a brief oxidation period, the NP was subjected to two stepwise TNP ramps from 1200 -2000 K under inert conditions, and then a second 1200 K oxidation period was used to probe changes in the reactivity.