By substitution of the Mn and Co cations in different sites in the spinel structure of Mn3O4 and Co3O4, by other cations which are not active in the selective reduction of nitrobenzene to nitrosobenzene, the role of these sites in the catalytic reaction can be studied. By combining activity measurements with quantitative surface analysis by low-energy ion scattering, which only probes the outermost atomic layer, the catalytic performance can be related to the surface composition. The results suggest that octahedral sites are exposed almost exclusively at the surface of the spinel oxide powders and that only these sites participate in the reaction. The only two low-index planes of the spinel structure which can satisfy this condition are identified as B(111) and D(110). Although the present results pertain to Mn3O4 and Co3O4, it is believed that the absence of occupied tetrahedral sites at the surface is a more general property of spinels.