The role of electrons and holes in the electronically excited oxidation of adsorbed CO on TiO2(110) has been investigated by tuning the surface electron and hole availability by the adsorption of Cl-2 or O-2. The presence of an electron acceptor (Cl-2 or O-2) on the TiO2(110) surface causes upward band bending, increasing the excited hole availability and decreasing the excited electron availability in the near surface region. This enhances O-2 desorption and depresses CO2 production during electronic excitation. This result gives clear evidence for the first time that the electronically excited CO oxidation reaction is caused by an electron-mediated process in contrast to O-2 desorption which is mediated by holes.