Direct (non-mediated) electrochemical oxidation of ammonia on boron-doped diamond (BDD) electrode proceeds mainly at high pH (>8) via free ammonia (NH3) oxidation. To enhance ammonia oxidation on BDD at low pH (<8), where mainly ammonium (NH4+) is present, oxidation of ammonia was mediated by active free chlorine. In this process, electro-generated in situ active chlorine rapidly reacts with ammonia instead of being further electro-oxidized to chlorate at the electrode surface. Thus, active chlorine effectively removes ammonia from an acidic solution, while the formation of by-products such as chlorate and possibly perchlorate is minimized. (C) 2010 Elsevier B.V. All rights reserved.