AbstractFulminic acid (HCNO) is an important intermediate in the NO re‐combustion process, which removes NOx from fossil fuel combustion. The reactions of HCNO with atoms, molecules, and free radicals in the gas phase have been studied using the DFT‐B3LYP method for structure optimization, while single‐point energies were calculated using the CCSD(T) method. The results indicate that an atom or free radical can easily combine with the carbon atom in HCNO, forming an intermediate state of approximately 45 kcal/mol below the reactants. This intermediate can undergo isomerization and/or decomposition to yield various products. Additionally, the atom or free radical (X) can abstract a hydrogen atom from HCNO to form CNO + HX, or add to the oxygen or nitrogen atom in HCNO to form HCNOX or HCN(X)O, respectively; these additional pathways are often ignored due to their high energy barriers. Similarly, the reaction of HCNO with stable molecules is significantly more difficult due to high energy barriers. The geometrical structures of the relevant species and the reaction enthalpies along the reaction pathways are in good agreement with available experimental data. Rate constants were calculated in a wide range of temperatures and pressures using TST, VTST, and RRKM theories. This work provides an overview of the reaction mechanisms, thermodynamic parameters, and rate constants for reactions between HCNO and atoms, molecules, and free radicals in the gas phase, contributing to both theoretical and experimental studies of this important species in practical applications.