In the reaction between NO and O-atoms, the concentration of NO remains essentially unaltered. The reaction can therefore be considered as a NO-catalyzed recombination of O-atoms. Its rate can be conveniently studied by following the disappearance of O-atoms by an isothermal calorimetric technique. The reaction was found to be third order, first order in the concentrations of O, NO, and M, where M is some third body. The third-order rate constant was found to be 1.85 × 1016 cc2 mole−2 sec−1 when M = O2, A, or He and 2.0 × 1016 cc2 mole−2 sec−1 when M = CO2 The rate constant was found to have a slight negative temperature coefficient which corresponded to a negative Arrhenius activation energy of about 0.2 kcal/mole. A detailed mechanism for the reaction has been proposed.