A numerical study has been conducted to clearly grasp the impact of chemical effects caused by added CO2 and of flame location on flame structure and NO emission behaviour. Flame location affects the major source reaction of CO formation, CO2 + H --> CO + OH and the H-removal reaction, CH4 + H --> CH3 + H-2. It is, as a result, seen that the reduction of maximum flame temperature due to chemical effects for fuel-side dilution is mainly caused by the competition of the principal chain branching reaction with the reaction, CH4 + H --> CH3 + H-2, while that for fuel-side dilution is attributed to the competition of the principal chain branching reaction with the reaction, CO2 + H --> CO + OH. The importance of the NNH mechanism for NO production, where the reaction pathway is NNH --> NH --> HNO, is recognized. In C-related reactions most of NO is the direct outcome of (R171) and the contribution of (R171) becomes more and more important with increasing amount of added CO2 as much as the reaction step (R171) competes with the key reaction of thermal mechanism, (R237), for N atom. This indicates a possibility that NO emission in hydrogen flames diluted with CO2 shows less dependent behaviour upon flame temperature. Copyright (C) 2004 John Wiley Sons, Ltd.