The influence of ad-atom layers of Pb, Tl, Cu, Bi and Ag, deposited in the underpotential region, on the reduction of 2-nitroimidazole on the Au(111) single crystal electrode was studied in aqueous acid solutions. It was found that the kinetics and mechanism of the nitro group reduction depend on the degree of coverage and the superlattice structure of the ad-layers. On Au(111) covered by a low coverage of disordered ad-atoms, the reduction follows an electrocatalytic mechanism associated with a four-electron reduction to 2-hydrohylaminoimidazole. At higher coverage where the ad-atoms form ordered monolayers, the reaction follows an electronation-protonation mechanism associated with a two-electron reduction to a N,N'-dihydroxyamine. This product, rapidly stabilized by two intramolecular hydrogen bonds, cannot be reduced further. Of particular interest is the effect of Cu upd for which a stepwise decrease of the reduction current was observed at potentials closely related to those of the peaks associated with the formation of Cu adlattice structures on Au (111) in the absence of 2-nitroimidazole.