A series of mixed cobalt aluminum oxides with spinel structure has been prepared in such a way that the concentration of cobalt ions in the surface increases gradually. The crystal structure does not change with the changing amounts of Co and Al. The catalytic activity of the cobalt aluminum oxides was tested in the selective reduction of nitrobenzene to nitrosobenzene, The rate of nitrosobenzene formation increases almost linearly with increasing Co concentration in the surface, This behavior is in agreement with the Mars-van Krevelen mechanism, which requires for its operation cations with variable valencies in the catalyst surface.