The flux of CO exhibits a temperature-dependent hysteresis, as seen in Figure . Such a hysteresis has been observed in the past by Olander et al. in the oxidation of pyrolytic graphite by O2 , and it was also observed in our previous work on vitreous carbon . Comparing the TD-prompt fluxes of CO and O, we see that the TD-prompt flux of O has the opposite trend of TD-prompt CO. This holds true for FiberForm (Figures and) and vitreous carbon (Figures and). Hysteresis arises because the surface takes a long time to reach a steady coverage and 25 minutes at each surface temperature is insufficient to reach steady coverage. To analyze the effect of hysteresis, we compute the flux difference, (COincrease -COdecrease) which is shown in Figure for FiberForm and vitreous carbon. At high surface temperatures, the surface oxygen coverage would be expected to reach a minimum under steady-state conditions. A higher difference would imply that the surface has lower coverage of O and is closer to steady-state conditions. As seen in Figure , the difference is higher for vitreous carbon, consistent with our hypothesis that vitreous carbon has a lower coverage of O atoms. The difference in the total thermal flux of O between the two temperature curves is also shown in Figure . A quantitative comparison cannot be performed because the fluxes have not been corrected for the massdependent detection sensitivity of our detector. However, qualitatively, the flux difference of O exhibits the opposite trend compared to CO.