This work has studied the adsorption and electrolytic reaction of ammonia at an IrO2 anode with a change of the pH, and it has evaluated the decomposition rate to nitrogen, the change of the byproducts, and the effect of the chloride ion on the decomposition reaction. Also, the electrolytic decomposition mechanism of ammonia was suggested. The ammonia in the basic solution was oxidized mainly to nitrogen gas through a direct electrolytic reaction accompanying an adsorption at the electrode where the oxygen evolution rarely occurred. The ammonium ion in the neutral and acid solution could be partly decomposed to nitrogen by the CH radicals generated in the condition of oxygen evolution, not by a direct electrode reaction. However, the decomposition rate was much lower than that in the basic condition. The hypochlorite ion generated from the chloride ion by the electrode was very effective for the decomposition of the ammonia and ammonium ion to nitrogen. The ammonium ion by hypochlorite in the neutral condition was changed to NO, as well as to nitrogen. (c) 2005 Elsevier Ltd. All rights reserved.