Photoinitiated unimolecular decomposition on a barrierless potential energy surface (PES) has been studied for the reaction NO2 --> O(P-3(2)) + NO(X(2)Pi(1/2), upsilon = 0) for excess energies up to approximately 17 cm(-1) above the dissociation threshold (i.e., D-0 for nonrotating molecules) by using expansion-cooled samples and the time-resolved pump-probe technique. To examine the threshold region with enough energy resolution to discern abrupt changes in the rate constant, should they occur, a pump-probe cross-correlation temporal width of similar to 25 ps and a pump line width less than or equal to 2 cm(-1) has been used. These are the first direct observations of the reaction rate constants in this energy regime. The rate constant was found to increase by an order of magnitude, varying from similar to 2 x 10(10) s(-1) to greater than or equal to 10(11) s(-1), the latter being a rough lower bound imposed by the experimental arrangement. The rate constant does not display the energy dependence predicted by using phase space theory, at least in detail. Rather, it appears to reflect the highly complex nature of the levels and the multiple PESs that are believed to be responsible for the anomalously high vibronic level density which has been observed just below D-0. These results bridge the gap between spectroscopic studies which have been carried out at energies just above D-0 and ultrafast experiments which have measured rate constants in this energy region with pump laser spectral widths of similar to 30 cm(-1).