Kinetics of the thermal decomposition of pentaerythritol tetranitrate (PETN) in condensed state has been investigated by high temperature infrared spectroscopy (IR) and thermogravimetry (TG) in conjunction with pyrolysis gas analysis, differential thermal analysis (DTA) and hot-stage microscopy. Kinetics of thermolysis has been followed by IR after suppressing volatilization by matrixing and by isothermal TG without suppressing volatilization to simulate actual user conditions. The best linearity was obtained for Avrami-Erofe＇ev equation, n = 1, in IR and isothermal TG. Activation energy was found to be 152 kJ mol(-1) and log A (in s(-1)) 16.96 by IR. The effect of additives on the initial thermolysis of PETN has been studied. Evolved gas analysis by IR shows that NO2, H2CO are produced in the initial stage of decomposition followed by NO, N2O, CO2, HCN and H2O. The decomposition in KBr matrix shows relative preferential loss in NO2 band intensity which indicates that the rupture of O-NO2 bond is the primary step in the thermolysis of PETN.