The rate of the reaction between nitrogen dioxide and fluorine to form nitryl fluoride has been measured at 27.7, 50.4, and 70.2°. Light absorption by nitrogen dioxide was used to follow the reaction. By using a 22-liter Pyrex flask and by going to low concentrations of reactants, between 0.5 and 20×10−8 moles cc−1, the rate of this fast reaction was reduced to a convenient value. The rate was found to be first order in each reactant, and it showed no dependence on nitryl fluoride. The proposed mechanism is NO2+F2→ lim k1NO2F+F (slow),NO2+F+M→ lim k2NO2F+M (fast).The empirical second order rate constant is k1, that of an elementary bimolecular reaction. If the rate constant is written as Ae−E/RT then A is 1.6×1012 cc moles−1 sec−1 and E is 10.5 kcal.