The thermal reaction of HNCO with NO2 has been studied in the temperature range of 623 to 773 K by FTIR spectrometry. Major products measured are CO2 and NO with a small amount of N2O. Kinetic modeling of the time resolved concentration profiles of the reactants and products, aided by the thermochemical data of various likely reactive intermediates computed by means of the BAC‐MP4 method, allows us to conclude that the reaction is initiated exclusively by a new bimolecular process: (Formula Presented.) with a rate constant, k1 = 2.5 × 1012e−13,100/T cm3/mols. The well‐known bimolecular reaction (Formula Presented.) is the only strong competitive process in this important reactive system throughout the temperature range studied. Kinetic modeling of NO formation and NO2 decay rates gave rise to values of k10 which were in close agreement with literature data. © 1993 John Wiley & Sons, Inc.