To investigate the reaction chemistry of HCN oxidation, a modeling study was performed. The plug flow calculation code was used at atmospheric pressure in the temperature range from 1000 to 1400 K. The effect of initial H 2O concentrations and that of other components were discussed. The oxidation of HCN is controlled primarily by the HCN + OH reaction in case of increasing H2O concentration. The oxidation of HCN starts at lower temperatures and the conversion of HCN to NO is inhibited by increase in H 2O concentration. N2O formation by the NCO + NO reaction is inhibited by increase in H2O concentration because of the small amount of NO and NCO. In the presence of initial NO, NCO acts as a reducing agent for NO. NCO mainly reacts with initial NO, so N2O formation is not affected by H2O concentration. In case of adding CO, CO oxidation chemistry acts as a source of a radical pool, and HCN oxidation shifts to lower temperatures. Increasing H2O affects, the consumption of O radical and inhibits NO formation. The effect of H2O concentration on N 2O formation is small because of the number of O and H radicals formed by CO oxidation.