The reduction of N-heterocyclic nitro compounds on Au, an sp metal elctrocatalyst, in acid solutions can follow an electrocatalytic and/or electron exchange mechanism. The electrocatalytic mechanism proceeds through chemisorption of the nitro group and reductive cleavage of one of the two N-0 bonds and gives diffusion-controlled limiting currents. The electron exchange mechanism proceeds through a dihydroxylamine which is reduced further after an irreversible loss of water to produce the nitroso intermediate and leads to kinetically controlled limiting currents. The strong electron-attracting protonated heterocyclic nuclei, as well as rendering the nitro group more easily reducible, help to stabilize the hydrated form of the nitroso intermediate. On Au/M(UPD) electrocatalysts the reduction mechanism depends on the UPD adatom coverage. On Au surfaces, with almost complete adatom layers, the reduction proceeds through the dihydroxylamine. However, on Au partially covered by adatoms the electrocatalytic mechanism appears to be the only reaction path for the reduction. A bridge adsorption complex of the nitro group [GRAPHICS] was assumed to explain its stronger chemisorption on Au partially covered by adatoms than on Au without adatoms.