Adsorbed bromine on Pt and Au electrodes acts as a catalyst for dissociative adsorption of H2O2 and S2O8-2, similar to adsorbed OH (and adsorbed iodine). This fact is revealed by analyses of newly found negative differential resistances and electrochemical oscillations, appearing in a potential region where the surface coverage of adsorbed bromine (theta(Br)) decreases steeply. The present result further supports the generality of a catalytic effect of adsorbed electronegative species such as OH, Br, and I on the dissociative adsorption of peroxides such as H2O2 and S2O8-2.