The solid-liquid equilibria (cryometry) formed during the thermal ageing of N,2,4,6-tetranitro-N-methylaniline (tetryl) with purities of 99.8 and 99.9 mol% were investigated using DSC. The mean rates and rate constants were determined for the temperature range from the start of melting to the chosen final temperature. For small degrees of tetryl conversion, alpha < 0.02, small changes in purity introduced considerable influences on the thermal decomposition rate. For tetryl(99.9 mol%), a strong catalytic effect on the thermal decomposition was observed for this degree of conversion. The calculated ratio of the unimolecular reaction rate constant and the catalytic reaction rate is k(1)/k(2) = 187. Oscillations of the mean rate values were obtained for the whole investigated range of conversion degrees below 0.06. The mechanism of the thermal decomposition of the initial stage is proposed.