The role of lattice oxygen atoms in the oxidation of nitrosobenzene to nitrobenzene has been established by using isotopically labeled oxides and, in other experiments, O-18(2) as oxidant, It appeared that with the most active catalyst, alpha-Mn3O4, the Mars and van Krevelen mechanism (oxidation by lattive oxygen atoms) is the most important mechanism, On gamma-Fe2O3 and gamma-Al2O3, disproportionation of nitrosobenzene prevails.