The thermal decomposition of dinitrogen pentoxide (N2O5→NO2+NO3) has been studied in excess nitrogen between 4¿1014 and 2.8¿1019 molecules cm-3 and between 262 and 307 K. This reaction is an example of unimolecular reaction kinetics, catalyzed by inert gases M, and exhibiting effects of non-equilibrium distributions over reacting energy states. Previous measurements of the limiting low-pressure second-order rate constant have been confirmed and extended to lower temperatures; with M=N2, the combined old and new data give k0'=6.1¿10-6 exp[(-9570¿200)/T> cm3 molecules-1 s-1. An estimate of the high-pressure first-order rate constant, k∞=l.78¿1017 exp[(-12540¿130)/T> s-1, was found to be lower than the previous value at 300 K, and the high-presssure activation energy was found to be larger than the previous estimate. The problem of extrapolating unimolecular reaction data to low temperature is not simple; a general theoretical formula is presented, which can be used approximately to extrapolate these data to stratospheric conditions.