During the simultaneous reduction of the surface oxide and deposition of a sub-monolayer of Ag on Pt electrodes, a non-equilibrium overlayer is formed. In such an overlayer, two Ag adatoms block one site available for hydrogen upd, independent of the surface coverage by silver. During relaxation of this nonequilibrium overlayer, blocking of hydrogen adsorption sites decreases. The formation of a surface alloy in the vicinity of imperfections, created by oxide reduction, is a primary step in the deposition of Ag. The surface alloy is also responsible for less than one-to-one blocking of hydrogen in the case of relaxed Ag overlayers.