In order to ascertain whether the presence of unstable species in the gas phase can influence the kinetics of a heterogeneous catalytic process, the oxidation of ammonia has been studied on a CoO catalyst both in the presence and in the absence of atomic oxygen in the gas phase. In accordance with previous results obtained for the same reactionon Pt-Rh catalysts, it is shown that the presence of oxygen atoms significantly lowers the activation energy of the process (from 30 to 9 kcal/mole). It is furthermore shown that oxygen atoms influence the selectivity of the catalyst in the sense of increasing the yield of NO with respect to N₂. From the observed kinetics one is lead to the conclusion that the slow step of the process is the reaction: $NH_{3} + O_{ads} \to \text{Intermediates}$ (NH₃O, NH, others) and that NO and N₂ are formed by successive reactions of the intermediates. The influence of oxygen atoms on the activation energy has previously been discussed for the catalytic oxidation on Pt-Rh gauzes. The influence of the oxygen atoms on the selectivity of the catalyst can be qualitatively understood on the basis of a simplified reaction scheme.