Because O2 oxidation is sensitive to the presence of undercoordinated surface sites, it serves as an excellent probe of changes in surface site distributions as NPs evolve under heating and etching conditions. Every type of carbon NP studied was found to evolve at high TNP under oxidizing conditions, eventually becoming inert to O2, and Fig. showed that the evolution also can occur under inert conditions, but much more slowly. Fig. uses EEO2 to track the evolution of NPs of all the carbon materials under oxidizing conditions. In Fig. EEO2 is plotted against M/Minitial, facilitating comparisons between different size NPs; the absolute masses are given in the figures indicated in the legend. Because evolution is also affected by time, Fig. plots EEO2 vs. reaction time. As in Fig. , squares indicate EEO2 values averaged over time periods with near-constant rates, and squares with downward arrows indicate periods when EEO2 dropped below our sensitivity limit.