The role of catalyst and the reason for the preferential formation of N 2 in the catalytic oxidation reaction of ammonia in water over a Ru (3wt.%)/TiO 2 catalyst were elucidated. It was verified that the catalyst in the reaction had no direct relevance to the selective formation of N 2 , but was responsible only for the oxidation of aqueous ammonia, NH 3 (aq), finally giving a molecule of nitrous acid. The preferential production of N 2 was experimentally demonstrated due to the homogeneous aqueous phase reaction of the nitrous acid-dissociated NO2- with NH4+ ions. Even under the highly oxidizing condition, NO2- was much more likely to react with NH4+ to form N 2 than being oxidized over the catalyst to NO3- as long as NH4+ was available in solution.