The equilibrium geometries, the bond dissociation energies, harmonic vibrational frequencies, and infrared intensities of peroxyacetic nitric anhydride (PAN), peroxyacetyl radical (PA), and NO2 which are important species in environmental chemistry, have been studied using high level of ab initio calculations. The global minimum energy conformation of the PAN molecule is found to be s-cis-PAN. The; predicted enthalpy and entropy changes at 1 atm and 298.15 K for the thermal decomposition of PAN are 25.8 kcal/mol and 42.3 cal/(mol.K), respectively. These values are in excellent agreement with the experimental values. A characteristic IR band of PAN appears at 1232 cm(-1), which is a CH3 rocking mode coupled with CO and OO stretches. The corresponding band of the PA radical appears at 1216 cm(-1), about 10 cm(-1) lower than that of PAN. Thus, determining the concentrations of PAN based on only this band would not be so reliable. We find that the PA radical has a strong IR band at 1150 cm(-1) due to the O-O stretch mode, while the PAN molecule has the corresponding band at 977 cm(-1). Since PAN and the PA radical do not have other IR bands at this region, this characteristic band could be used for identification and measurement of the PA radical and thus for spectroscopic study of thermal decomposition mechanism of PAN.