The kinetics of thermal decomposition of 3-nitro-2,4-dihydro-3H-1,2,4-triazol-5-one (NTO) in the temperature interval from 200degreesC to 260degreesC was investigated using a glass Bourdon gauge. The overall decomposition reaction includes two distinct stages: the fast first-order decomposition and the subsequent autocatalytic reaction. The importance of the first stage increases with increasing decomposition temperature and decreasing loading density of the Bourdon gauge (m/V). A period of preliminary heating, at a lower temperature, strongly influences the autocatalytic stage when the decomposition is carried out at a higher temperature. In the temperature domain 200-220degreesC, the Arrhenius constants of the decomposition reaction are found to be close to the values usually observed for nitrocompounds: E = 173 kJ/mol and log(10) k approximate to 12.5 (s(-1)). It is shown that a simple model of NTO decomposition based on an autocatalytic reaction of the m-th order can describe the course of the decomposition at high temperature but the in number appears to be excessively high, up to 4. A new model of the decomposition is developed, including an initial monomolecular reaction, decomposition of the crystalline substance, and an autocatalytic reaction of NTO dissolved in liquid decomposition products. This model gives the common order of autocatalysis, m = 1.