An effect of oxygen species formed from O-2, N2O and NO on the selectivity of the catalytic oxidation of ammonia was studied over a polycrystalline Pt catalyst using the temporal analysis of products (TAP) reactor. The transient experiments were performed in the temperature range between 773 and 1073 K in a sequential pulse mode with a time interval of 0.2 s between the pulses of the oxidant (O-2, N2O and NO) and NH3. In contrast to adsorbed oxygen species formed from NO, those from O-2, and N2O reacted with ammonia yielding NO. It is suggested that the difference between these oxidising agents may be related to the different active sites for dissociation of O-2, N2O and NO, where oxygen species of various Pt-O strength are formed. Weaker bound oxygen species, which are active for NO formation, originate from O-2 and N2O rather than from NO. These species may be of bi-atomic nature. (c) 2006 Elsevier B.V. All rights reserved.