Structures of ordered adsorbates have been determined during the course of the electrocatalytic reduction of oxygen by using synchrotron surface X-ray scattering techniques. The origin of the catalytic action of Tl adlayers on O-2 reduction on Au has been determined, and a possibility of measuring the adsorption configuration-specific activity in O-2 reduction has been demonstrated in the reaction on Ag(100) with Br adlayer. On the Au(111) electrode surface, the oxidation/redeposition of T1 in catalyzing the four-electron oxygen reduction is revealed from the analysis of X-ray diffraction intensity as a function of the O-2 reduction current for the (3 x root 3)-2TlBr adlayer. This indicates that the redox property of the metal adsorbates is responsible for the catalytic effect. On Ag(100) with the c(2 x 2) Br adlayer, the O-2 reduction current resulting solely from adsorption in the "end-on" configuration through the 4-fold symmetry holes in the c(2 x 2) Br lattice is observed, showing that the "bridge" adsorption may not be necessary for the four-electron O-2 reduction on Ag at large overpotentials.