The promoting mechanism of alkali ions on Mn3O4 in the selective reduction of nitrobenzene has been investigated using X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy and ion scattering spectroscopy. It appeared that the alkali ions accumulate on the outermost surface layer, but do not change the oxidation state of manganese. Heating of the fresh catalysts in ultra-high vacuum results in a disappearance of the alkali ions from the surface. This shows that the alkali ions migrate easily over the surface at elevated temperatures. However, when the catalysts have been treated with nitrobenzene, the same heating procedure does not result in a disappearance of the promoter ions. Moreover, it can be seen that the surface has been covered with adsorbates derived from nitrobenzene. It can be concluded that the promoting effect is not due to the electronic structure changes in the interior of the Mn3O4 catalyst, but is rather due to one of the following mechanisms : (a) an electrostatic field effect of alkali cations or (b) the formation of intermediates with the reaction components, in the adsorbed layer.