Numerical simulation with detailed chemistry has been carried out to clearly discriminate the thermal and chemical contributions of added diluents (H2O and CO2) to major flame structures and NO emission characteristics in H-2/N-2 counterflow diffusion flame. The pertinence of GRI, Miller-Bowman, and their recent modified mechanisms are estimated for the combined fuel of H-2, CO2, and N-2. A virtual species X, which displaces the individual CO2 and H2O in the fuel sides, is introduced to separate chemical effects from thermal effects. In the case of H2O addition the chain branching reaction, H + O-2 --> O + OH is considerably augmented in comparison with that in the case Of CO2 addition. It is also seen that there exists a chemically super-adiabatic effect in flame temperature due to the breakdown of H2O. The reaction path of CH2O --> CH2OH --> CH3 and the C1-branch reactions become predominant due to the breakdown of CO2. In NO emission behaviour super-equilibrium effects caused by the surplus chain carrier radicals due to the breakdown of added H2O are more superior to the enhanced effects of prompt NO with the breakdown of added CO2. Especially, it is noted that thermal NO emission is directly influenced by the chemical super-equilibrium effects of chain carrier radicals in the case of H2O addition. As a result the overall NO emission in the case of the addition of H2O is higher than that in the case Of CO2 addition. Copyright (C) 2002 John Wiley Sons, Ltd.