Ammonia is a major source of water pollution. One of the most common methods for removal of ammonia from water is oxidation. In this work, ozonation of ammonia using microbubbles was studied in a pilot plant. The experimental results indicate that ozone microbubbles were quite effective in oxidizing ammonia. Oxidation of ammonia was effective at high pH and high ozone generation rates. Ozonation was found to occur by direct reaction of ozone with ammonia at the higher pH. However, the hydroxyl radicals were also involved at the lower pH. Bromide ions acted as a catalyst in the ozonation process, and a faster rate of oxidation of ammonia and lower yield of nitrate was observed. The volumetric mass transfer coefficient of ozone in water was determined. It increased with the increasing rate of ozone generation and the pH of the medium.