The role of band bending on the efficiency of charge transfer across the TiO 2 (110) single crystal surface has been measured in ultrahigh vacuum, in the absence of a wide range of surface site inhomogeneities, surface impurities and solvent effects, and particle size effects. The adsorption of the Cl 2 (electron acceptor) molecule and the O 2 (electron acceptor) molecule have been found to enhance hole transport from TiO 2 to 18 O 2 molecules adsorbed on oxygen vacancy sites, increasing the rate of electron stimulated desorption (ESD) of 18 O 2 . This confirms that O 2 -ESD is hole mediated. Conversely, adsorption of CH 3 OH, a donor molecule, reduces the transfer rate for holes to the adsorbed O 2 , reducing its rate of ESD to near zero. The maximum effect of donor and acceptor molecules occurs near 1 monolayer coverage.