Abstract: The effects of gas compositions on NOx reduction and NH₃ slip by selective non-catalytic reduction (SNCR) with NH₃ were investigated in a simulated cement precalciner atmosphere. The results show that the presence of H₂O improves NOx reduction and widens the reduction temperature window significantly. O₂ is indispensable for reducing NOx. The optimum reduction temperature decreases and the temperature window widens to a lower temperature with the increase of O₂ content. In addition, the increase of O₂ content also results in a decrease of the maximum NOx reduction efficiency. The effect of SO₂ on NOx reduction is negligible in the simulated precalciner atmosphere. To increase CO concentration makes NO reduction take place at relatively low temperatures. However, NH₃ will tend to be oxidized into NO instead of reducing NO after entering the stream containing O₂ at high temperatures if it is initially blended with a high concentration of CO in an oxygen-free environment. The increase of H₂O, O₂, SO₂ or CO concentration is helpful to reduce NH₃ slip in the temperature region below 900 °C. These effects are resulted from the fact that the generation and consumption of O and OH radicals which are crucial to NO reduction and formation can be influenced by the four gas compositions. In industrial operation of SNCR for cement precalciner, these effects should be taken into account to increase NOx reduction efficiency and avoid NH₃ slip.