A cobalt oxide catalyst for ammonia oxidation of high and stable activity and selectivity has been obtained and tested in a fluidized bed laboratory reactor. Its active component is Co3O4 It has been found that much less nitrous oxide is produced in ammonia oxidation on this catalyst than on platinum. The macrostructure of the cobalt catalysts, and especially the presence of mesopores of diameter <0.1 mu m have an essential effect on their activity and selectivity and on the stability of these properties. In the macrostructure of selective and stable catalysts the share of the mesopores <0.1 mu m is extremely low while the total pore volume amounts from some tens to a hundred and some tens of mm(3)/g. A hypothetical model has been presented to explain the effect of the catalyst＇s macrostructure on its properties.