Surface X-ray scattering has been used to determine the structure of Tl adlayers on the Au(111) electrode surface during the course of O-2 reduction in acid solution. O-2 reduction is considerably catalyzed by Tl adlayers on Au(lll). The half-wave potential is shifted to more positive values in the presence of the Tl adlayer. In the potential region between -0.18 and -0.4V (vs. see), the reaction mechanism changes from a 2e-reduction on Au(111) to a 4e-reduction on Au(111) covered with a low-coverage Tl phase. The close-packed rotated-hexagonal Tl phase, which exists in the potential range between -0.4 V and the bulk Tl deposition at similar to -0.7 V, has a lower activity for O-2 reduction than the low-coverage phase. O-2 reduction does not change the Tl coverage in this phase but causes a significant decrease of the in-plane diffracted intensity. This observation indicates that the O-2 molecules interact directly with the Tl adatoms prior to the charge transfer. It provides the most direct evidence that the outer sphere charge transfer mechanism is not operative for some surfaces. H2O2 reduction is facile on the surface covered with the low-coverage Tl phase, while it is almost completely suppressed by the rotated-hexagonal phase.