Carbon monoxide oxidation over oxidized Rh surfaces is known to be sensitive to both the oxygen species present as well as the surface temperature. Although CO oxidation on Rh(111) is a prototypical heterogeneously catalyzed oxidation reaction, questions remain about the reactivity of the individual oxygenaceous phases present on the surface. For example, the effects of surface temperature or the duration of CO exposure have not been previously determined on the oxygen-rich, (2 × 1)-O surface adlayer. In this paper, we present results from a study that used a combination of ultra-high vacuum surface science techniques to measure the oxidation of CO by oxygen in the (2 × 1)-O adlayer. The surface temperature during the CO exposure was varied between 100 K and 350 K, and the effect of the surface temperature on CO oxidation was determined for CO exposures between 5 L and 300 L. We observed that the surface temperature had little effect on the CO2 yield or the amount of residual oxygen for CO exposures up to 300 K, but these quantities were reduced after CO exposures above 300 K. We also found that CO oxidation was unchanged by the extent of the CO exposure at both 300 K and 350 K. Taken together, these results show that CO was oxidized over the (2 × 1)-O adlayer during CO exposures above 300 K via a different reaction pathway than the one followed by co-adsorbed O and CO in the (2 × 2)-2O+CO adlayer and that these lower-barrier reactive sites were not regenerated during the CO exposure.