The catalytic behaviour of the PbO-Mn3O4 and the Bi2O3-MoO3 systems was investigated in the selective reduction of nitrobenzene to nitrosobenzene. Lead compounds appeared to be good catalysts, and co-catalysts when used with Mn3O4. Different from oxidations by dioxygen, Bi2O3 alone is a good catalyst and formation of mixed Bi-Mo-O compounds does not enhance the catalytic activity. It is suggested that the difference between these catalysts in the mentioned reaction is related to the way in which the oxygen vacancy is represented by the oxygen donor.