The oxidation of ammonia in liquid phase has been studied over Pt supported on graphite, carbon, TiO2, and ZrO2. The activity of the Pt catalyst was compared with the activity of Pd, Ru, and Ir catalysts. The reaction was studied in a high-pressure continuous stirred tank reactor in the temperature range of 150¿180°C and at a pressure of 1.5 MPa. It was found that Pt supported on TiO2 and ZrO2 was less active compared to Pt supported on graphite due to the lower dispersion of platinum. Ir supported on TiO2 was more active and selective to nitrogen than Pt/TiO2. Pt showed a higher activity compared with Pd and Ru, but a lower selectivity to nitrogen. The selectivity and activity of the Pt/graphite catalyst depend strongly on the concentration of oxygen in the gas phase. Nitrogen and water are the only products when the reaction proceeds at oxygen gas/liquid mass transfer limitation conditions. In surplus of oxygen, N2O and NO2- were detected as products of further oxidation. N2O is the second major product besides nitrogen. With decreasing temperature and increasing concentration of oxygen in the gas phase, deactivation of the catalyst occurred. The reaction start-up procedure significantly influences the activity of the catalyst.