In this study the ability of various acid catalysts to selectively oxidize NH3 to N2 was examined in synthetic gasification gas, when O2 and NO simultaneously or O2 or NO alone were added to the gas. If no oxidizers were added to the gas, all catalysts were inert and no reduction of NH3 took place. With aluminum oxide 95% NH3 reduction took place, when O2 and NO were added simultaneously to the gas at 400 °C. Also aluminum silicates and montmorillonite were effective. Generally, NH3 conversion was always decreasing when the temperature was increasing. At low temperature, N2O formation was detected, which was retarded at high temperature. Only a small amount of HCN was formed, when temperature was above 600 °C. Adding NO alone to the gas did not reduce the NH3 content of the gas. With O2 addition alone, 50−60% NH3 reduction was achieved at a very narrow temperature window of 600−700 °C.