The influence of foreign gas additions (argon, nitrogen, carbon dioxide, carbon tetrafluoride and mixtures of them) on the thermal decomposition of nitrous oxide at a series of different initial pressures has been studied. The curves of k, the formal first-order constant, as a function of x, the foreign-gas pressure, show regions of rapidly falling slope analogous to those found in the curves of k against n, the initial pressure of nitrous oxide. The forms of the curves have been investigated in some detail, and suggest very strongly the existence of potentially rate-determining processes other than those normally assumed in unimolecular reactions (which are energization of molecules in collisions and chemical decomposition of these molecules). It is now postulated that spontaneous and collision-induced transfers of energized nitrous oxide molecules to triplet states constitute the processes in question, and on this basis the forms of the k, n and k, x curves are interpreted. This postulate links up with certain spectroscopic considerations previously advanced by Herzberg.