Oxidative decomposition of NH4* (10 mmol L-1) with 03 in water was studied at 333 K over a variety of metal oxide catalysts without pH control of the solution. Although MgO and NiO had the highest catalytic activities, large amounts of undesired NO3- formed due to low selectivity to gaseous products as well as high activity. Co304, which was slightly less active than MgO and NiO, was the best catalyst in terms of activity, selectivity to gaseous products, and dissolution degree among the metal oxide catalysts studied. Over Co304, NH4* was selectively oxidized to N2 with 88% selectivity in water, and the dissolution degree of Co304 was less than 1%. Fe203, Sn02, Mn304, CuO, MgO, and Al203 were less selective to gaseous products or much less active for the reaction. The selectivities to gaseous products were strongly related to the standard enthalpy changes of formation per mol of oxygen atom ( AH'f) of the metal oxides. The metal oxide catalysts with low AH'f, like Co304, showed high selectivity to gaseous products probably due to the low surface density of the active oxygen formed from 03 on the catalysts. Chloride ions (Cl present in the reaction solution significantly accelerated the reaction rate for NH4' decomposition with 03 in the presence of Co304. This was due to the involvement of Cl- in the catalytic cycle. For instance, C10-, which may form by the reaction of Cl- with 03 over Co304, could further oxidize NH4*. 2013 Elsevier B.V. All rights reserved.