Abstract This work aimed at deeply investigating the effects of H2O and CO2 on NO formation under volatile-N combustion. The characteristics and mechanism of NO formation during volatile-N model compounds (pyridine and pyrrole) combustion in H2O/CO2 atmosphere were systematically investigated via experiments and numerical simulations. The results both showed NO formation significantly decreased with addition of H2O and CO2 during combustion of pyridine and pyrrole. Under O2/H2O atmosphere, H2O can significantly promote the reaction H + O2(+M) ↔ HO2(+M) which consumed H and O2 in reaction system. As a result, the chain reaction of H + O2 ↔ OH + O was inhibited and O radical decreased, thus NO formation was inhibited. Under O2/CO2 atmosphere, addition of CO2 inhibited CO + HO ↔ CO2 + H and NCO + O2 ↔ NO + CO2. As a result, NO reduction was promoted via 2CO + 2NO ↔ N2 + 2CO2 and oxidation of NCO to form NO was inhibited. Under O2/H2O/CO2 atmosphere, NO formation was further inhibited due to inhibitions of HCN + OH ↔ CN + H2O and NCO + O2 ↔ NO + CO2. The mechanism of nitrogen conversion was found: Nitrogen in pyridine and pyrrole was firstly pyrolyzed into NH3 and HCN, then oxidized by O and OH groups to be nitrogen-containing intermediate phases such as NH, HNO, N, NCO, etc. HNO and NCO will continue to be oxidized to generate NO. NCO, NH and NH2 would react with the generated NO to form N2, and NCO dominated.