of CO and O that increase with decreasing surface oxygen coverage . The desorption barrier shifts the TOF distributions to shorter times than predicted by a MB distribution at the surface temperature. However, TD TOF distributions obtained from FiberForm and vitreous carbon were fit well under the assumption of a MB distribution for the thermal desorption processes, suggesting little or no barrier to desorption. It is likely that oxygen surface coverage on FiberForm, and perhaps on vitreous carbon, would be much higher than on HOPG, resulting in lower energy barriers for CO desorption. On FiberForm, the likelihood for O atoms to chemisorb on the surfaces of individual fibers is high as a result of the potential for many bounces in the fiber network. In addition, the carbon surfaces on FiberForm and vitreous carbon presumably have myriad defects that would lead to increased sticking of O atoms on these surfaces compared to the much more ordered surface of HOPG. Indeed, the reactivity of HOPG is significantly lower than that of vitreous carbon or FiberForm, as indicated by the relatively low signals of CO from HOPG and the small fraction of TD O atoms. Thus, surface oxygen coverage must be playing a significant role in the fate of oxygen atoms that bombard the different types of sp 2 carbon surfaces. The higher oxygen coverage on FiberForm and vitreous carbon opens many pathways for CO desorption following relatively long surface residence times, such as those that we have observed. adsorption on a carbon surface is very low . Hence, TD-slow1 and TD-slow2 cannot be attributed to the three-dimensional network of FiberForm alone. In the previous work on vitreous carbon , it was suggested that the slow signals of CO observed at 800 K might be attributable to the Boudouard reaction, where CO2 reacts with the carbon surface to form CO. Typical activation energies reported in the literature for Boudouard reactions are greater than 100 kJ mol -1 . Although the mechanism for a Boudouard reaction is not well understood, the ratelimiting step typically involves the abstraction of adsorbed O by CO2 from the surface .