In this study the ability of aluminum oxide to catalyze the selective oxidation of NH3 in hot pressurized (20 bar) synthetic gasification gas was examined. When the mixture of NO and O2 was used as oxidizer, it was found that part of the NO may be converted to NO2 in the gas feeding system. It was found that the ratio of NO/NO2 in the oxidizer mixture can be controlled by controlling the residence time and the temperature of the gas feeding system. Increasing the pressure increased the rate of NO2 formation. When the oxidizer consisted of NO2 and O2, large NH3 conversion was attained below 500 °C. With NO and O2 addition into the hot gasification gas the highest NH3 conversion took place at 550 °C. The addition of O2 alone to the aluminum oxide bed at high pressure reduced also the NH3 content of the gas. It was shown that aluminum oxide catalyzes the reaction between NH3 and O2.