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