The effect of NH, NH₂, and HNO on NO_x and De‐NO_x chemistry of a NH₃/H₂/air mixture at a pressure of 20 bar is investigated. Results suggest that the increase in pressure reduces NO_x emissions and increases HO₂ radical production through the reaction H + O₂(+M) → HO₂(+M). In contrast with OH and O radicals, the HO₂ radical is less reactive, which prevents NO formation. The fuel‐bound NO_x emissions mainly depend on the reactions NH + OH → NO + H, HNO(+M) → NO + H(+M), HNO + OH → NO + H₂O, and HNO + O₂ → NO + HO₂, and thermal NO_x depends on the reactions N + O₂ → NO + N and N + OH → NO + H. At a pressure of 20 bar, the N₂O is further converted to NO₂ and N₂ through the reaction N₂O + NO → NO₂ + N₂. The abundance of HO₂ radicals at high pressure also initiates the conversion of NO to NO₂ via the reaction NO + HO₂ → NO₂ + H.