Numerical simulation with detailed chemistry has been carried out to clearly discriminate the thermal and chemical contributions of added diluents (H 2 O and CO 2 ) 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 , CO 2 , and N 2 . A virtual species X ; which displaces the individual CO 2 and H 2 O in the fuel sides, is introduced to separate chemical effects from thermal effects. In the case of H 2 O addition the chain branching reaction, H+O 2 ! O+OH is considerably augmented in comparison with that in the case of CO 2 addition. It is also seen that there exists a chemically super-adiabatic effect in flame temperature due to the breakdown of H 2 O. The reaction path of CH 2 O ! CH 2 OH ! CH 3 and the C1-branch reactions become predominant due to the breakdown of CO 2 . In NO emission behaviour super-equilibrium effects caused by the surplus chain carrier radicals due to the breakdown of added H 2 O are more superior to the enhanced effects of prompt NO with the breakdown of added CO 2 . 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 H 2 O addition. As a result the overall NO emission in the case of the addition of H 2 O is higher than that in the case of CO 2 addition.