Structures of Br adlayers were determined on the Ag(100), Au(100) and Pt(111) electrode surfaces during the course of O2 reduction by using synchrotron surface X-ray scattering techniques and correlated with the O2 reduction activity obtained from rotating disk electrode measurements. On Ag(100), the c(2×2) Br adlayer, precludes the O2 adsorption in ‘bridge’ configuration, and the O2 reduction current resulting solely from adsorption in the ‘end-on’ configuration through the four-fold symmetry holes in the c(2×2) Br lattice is observed at large overpotentials. On Au(100), O2 reduction is completely inhibited by the c(√2×2√2)R45° phase. The reaction takes place only at potentials negative of the low potential limit of the existence of that phase. O2 reduction is also completely blocked by the commensurate (3×3) Br adlayer on Pt(111) at high potentials. Below 0.5 V, the ordered phase vanishes and O2 reduction takes place. Adsorbed disordered Br adions change the mechanism of O2 reduction on Pt(111) into a 2e-reaction. O2 affects the stability of ordered Br adlayer on Pt(111), but not on Ag(100) and Au(100).