The photodissociation of NO3 was studied using the method of molecular beam photofragmentation translational spectroscopy. The existence of two photodissociation channels was confirmed under collision-free conditions. At excitation energies below D0(O-NO2) for internally cold NO3, we observe a large quantum yield (0.70 +/- 0.10 at 588 nm) for a concerted three-center rearrangement resulting in NO(2PI) + O2 (3SIGMA(g)-,1DELTA). The quantum yield for the NO + O2 channel decreased sharply at wavelengths shorter than 587 nm, falling to <0.01 at 583 nm, while the NO2 + O(3p) quantum yield increased to >0.99. On the basis of this wavelength dependence and the product translational energy distributions, we conclude that the wavelength threshold for NO3(0,0,0,0) --> NO2(0,0,0) + O(P-3(2)) is 587 +/- 3 nm, i.e. D0(O-NO2) = 48.69 +/- 0.25 kcal/mol. From the enthalpies of formation of O(P-3(2)) and NO2 (2A1), we calculate DELTAH(f)-degrees (NO3) = 18.87 +/- 0.33 kcal/mol at 0 K and DELTAH(f)-degrees (NO3) = 17.62 +/- 0.33 kcal/mol at 298 K. This is 2.23 kcal/mol higher than the most recent thermochemical value but is consistent with a value calculated indirectly using the most recent values for the electron affinity (EA) of NO3 and DELTAH(f)-degrees-(NO3-). From the wavelength dependence and translational energy distributions for NO3 --> NO + O2, the potential energy barrier for NO3(2A'2) --> NO(2PI) + O2 (2SIGMA(g)-) was found to be 47.3 +/- 0.8 kcal/mol.