A large discrepancy exists among recent estimates of the primary quantum efficiencies (ϕ1 ) of nitrogen dioxide photodecomposition, NO2 +h&ngr;→NO+O(3 P), in the wavelength range from 374 to 396 nm. To resolve this problem, quantum yields of formation of NO, O2 , and NO2 loss have been measured for NO2 vapor at low pressures (0.13--0.30 torr) irradiated at selected wavelengths (334.1--404.3 nm) and temperatures (273--370 K). From these data, estimates of ϕ1 were derived which confirm the previous findings of Jones and Bayes (1973b): ϕ1 increases rapidly from near zero at 424 nm to near unity for excitation at λ 397.9 nm (λdiss ) is made up in large part from the rotational and vibrational energy of the NO2 molecules. Recommended values for ϕ1 , based upon a review of these data and estimates made over a period of 58 years, are given as a function of wavelength. ¿American Geophysical Union 1987