An investigation has been made of the stationary supersonic nozzle flow of a reacting gas mixture of nitrogen tetroxide and dioxide carried at low concentration in nitrogen. A judicious choice of supply conditions and nozzle geometry made it possible to produce flows either in chemical equilibrium or in states between this and the frozen flow. The flow could be fully determined from pressure and area measurements and, by applying an equation for the reaction mechanism, rate constants of recombination could be found. The results of these measurements agreed with those obtained from independent optical absorption measurements, and the third‐order rate constant for recombination was found to be 3 × 1014cm6mole−2sec−1for 210 <T< 330°K. Finally, experiments with stationary oblique shocks led to a determination of the dissociation rate constant.