The active sites of the spinels Mn3O4 and Co3O4 in the selective reduction of nitrobenzene to nitrosobenzene have been investigated, by total and partial substitution of Mn-n+ (Co-n+) ions in tetrahedral position and/or Mn-n+ (Co-n+) ions in octahedral position by redox-inactive ions. Investigation of the catalytic activities of the totally substituted manganese spinel ZnMn2O4 and that of the totally substituted cobalt spinels CoAl2O4 and ZnCo2O4 showed that the manganese and cobalt ions in octahedral position were responsible for the activity of the reaction. MnAl2O4, however, also showed some activity for the selective reduction of nitrobenzene, which should be due to surface enrichment and/or oxidative transfer of manganese from tetrahedral sites to octahedral sites. XRD results of the partially substituted spinels MnxZn1-xAl2O4 showed that there was a linear correlation between the manganese concentration expressed by x and the lattice constant. Moreover, each sample showed segregation of Al2O3 : again there was a linear correlation between x in a certain series of MnxZn1-xAl2O4 and the amount of free Al2O3. This points to an oxidative transfer of manganese from tetrahedral sites to octahedral sites. XPS showed that the concentration of manganese in the surface layer corresponded roughly to the value x, but the concentration of zinc in the surface layers was much smaller that expected. The catalytic activities of the MnxZn1-xAl2O(4) series of catalysts are larger than would be expected from the results on ZnMn2O4 and ZnCo2O4.