Significant nitrous oxide (N20) emissions have been observed from coal and oil burning power plants, but not from industrial gas flames, even when they are doped with an equivalent amount of fuel nitrogen. The unknown mechanism by which this N20 forms has considerable importance in determining the influence of common and advanced nitrogen oxide (NOx) control strategies on N20. The present study uses both detailed kinetic modeling and plug-flow simulator experiments to investigate this mechanism. Kinetic modeling suggests that net N20 is unlikely to form within flame zones because of its rapid removal by N20 + H--'N2 + OH. Also, reduction of nitric oxide (NO) on coal char can be a significant N20 source only if the rates are many times their published values. However, both kinetic modeling and experiments show that if HCN appears in the gas phase between 1150 and 1500 K it will be partially converted to N20. The source of HCN may be either direct devola "tdization, or a char gasification reaction. This suggests that the N20 that appears in coal flames may originate from the homogeneous reaction of devolatilized char nitrogen in the fuel-lean postflame region. The kinetics indicate that the formation is principally through the reaction NCO + NO'-*N20 + CO.