Studies of the oxidation of ammonia have been carried out using the two-stage catalytic system, where the first stage consisted of PtRh5 platinum-rhodium gauze and the other of the 75 mm high bed of granulated Co3O4 catalyst. Measurements were carried out at a pilot unit at pressure of 0,46 MPa. It has been shown that the bed of cobalt catalyst can be use instead of 2 to 4 PtRh gauze. The cobalt catalyst, being deactivated in the process of ammonia oxidation, can be reactivated under the action of the reacting gas mixture of a higher ammonia conversion, i.e. after passing the PtRh gauze, or by controlling the proper temperature range. Activity and selectivity of the cobalt catalyst is strongly influenced by the pore size and the overall porosity of pellets.