Thin films of NTO (5-nitro-2,4-dihydro-3H-1,2,4-triazol-3-one) are subjected to transient pyrolysis using a pulsed infrared laser in order to determine the initial steps of the thermal decomposition mechanism under rapid heating conditions, After heating, the film is quenched to 77 K in about 2 ms, thereby trapping reaction products which are subsequently detected by transmission Fourier-transform infrared spectroscopy, The first observed product of the decomposition is CO2; no evidence has been found for formation of NO2 or HONO in the early stages of thermal decomposition. The experimental results show that the CO2 reaction product is formed by bimolecular oxygenation of the carbonyl group by the nitro group of an adjacent NTO molecule. This conclusion is contrasted with mechanisms proposed by other research groups based on slow heating experiments.