The reduction of nitric oxide over lime and char at fluidized bed conditions has been studied by many authors, but less attention has been paid to other surfaces existing in the boiler. This investigation concerns the reduction of nitric oxide by carbon monoxide, with or without oxygen present, and also the effect of small amounts of water. The reactions were studied in a fixed-bed quartz reactor with quartz, magnesium oxide, magnesium hydroxide, and dolomite as bed materials in the temperature interval 600-950 degrees C. The residence time and the concentrations of nitric oxide, carbon monoxide, and oxygen were varied. Magnesium oxide formed from heated magnesium hydroxide and calcined dolomite were more active than magnesium oxide of analytical grade, which indicates that the origin of the surface is important for the reaction. The reaction between nitric oxide and carbon monoxide is very sensitive to the presence of oxygen. When oxygen was present the reduction of NO almost ceased. However, if the gases were thoroughly dried, the effect of oxygen addition was not that severe.