The unimolecular decomposition of expansion-cooled NO3 has been investigated in the threshold regime of the NO+O-2 channel. Photoexcitation in the region 16780-17090 cm(-1) (596-585 nm) prepares ensembles of molecular eigenstates, each of which is a mixture of the B(2)E’ bright state and lower electronic states. The X(2)A(2)’ ground state is believed to be the probable terminus of (2)E’ radiationless decay, though participation of A(2)E " is also possible. For these photon energies, unimolecular decomposition occurs exclusively via the NO+O-2 channel, and NO yield spectra and state distributions have been obtained. The yield spectra are independent of the rotational state monitored, as expected for a large reverse barrier. The state distributions are insensitive to the photolysis photon energy and can be rationalized in terms of dynamical bias. The NO yield goes to zero rapidly above the O+NO2 threshold (17090+/-20 cm(-1)). Because of tunneling, the NO+O-2 channel does not have a precise threshold; the value 16780 cm(-1) is the smallest photon energy that yielded signals under the present conditions. Very small decomposition rates were obtained via time-domain measurements in which reactive quenching of long-lived NO3 fluorescence was observed. The rates varied from 1x10(4) at 16780 cm(-1) to 6x10(7) s(-1) at 16880 cm(-1), and their collision free nature was confirmed experimentally. These data were fitted by using a one-dimensional tunneling model for motion along the reaction coordinate combined with the threshold Rice-Ramsperger-Kassel-Marcus (RRKM) rate. The top of the NO+O-2 barrier is estimated to lie at 16900+/-15 cm(-1). Translational energy measurements of specific NO cm (X(2) Pi(Omega),upsilon,J) levels showed that O-2 is highly excited, with a population inversion extending to energies above the a(1) Delta(g) threshold, in agreement with previous It is possible that the main O-2 product is X(3) Sigma(-) though some participation of a(1) Delta(g) cannot be rulted out. Within the experimental uncertainty, b(1) Sigma(g)(+) is not produced.