A new way of forming HCN in flames via N2O and NNH reacting with CHi radicals is proposed and tested for rich and lean gaseous premixed flames of CH4 and air and also of CH4, N2O and Ar. This new route is thermodynamically more probable than Fenimore's direct reaction of N-2 with CHi radicals. In fact, it is shown that the new mechanism is more important than Fenimore's reaction in both rich and lean flames. Rate constants of the new reactions forming NO have been suggested on the basis of numerical modeling. It has been shown that the formation of NO through HCN is most effective as the result of reactions initiated by N2O + CH3 --> CH2NH + NO, followed by CH2NH + H --> H2CN + H-2 and CH2NH + O --> H2CN + OH. In flames of CH4 and air, a substantial source of N2O comes from the reverse of the reaction N2O + CH3 <----> CH3O + N-2 in the reaction zone. A formula based on the steady state assumption and partial. equilibrium limits the number of nitrogen conversion reactions to only 12; this was tested using a premixed flame of CH, and air. (C) 2003 The Combustion Institute. All rights reserved.