The mechanism of the electroreduction of peroxide on Bi-submonolayer-modified Au(111) surfaces is examined using surface-enhanced Raman scattering (SERS) measurements along with detailed density functional theory (DFT) calculations. The spectroscopy shows the presence of Bi-OH and Bi-O species at potentials just positive of that where peroxide is reduced. These species are not present in solutions absent either peroxide or Bi. DFT calculations show that peroxide is unstable relative to Bi-OH when Bi is present in the (2 x 2) configuration on Au(111) known from previous work to be catalytically active. The spacing between Bi adatoms is such that peroxide association with two Bi cannot occur without O-O bond cleavage. The full Bi monolayer is catalytically inactive and exhibits none of the Bi-OH or Bi-O signals seen for the active surface. The calculations show that as the Bi coverage becomes greater and the Bi adatom spacing becomes smaller, peroxide can adsorb on Bi without O-O bond rupture. These results indicate an important role for M-OH species in peroxide electroreduction.