The principal unimolecular dissociation pathways for PAN (peroxyacetyl nitrate, CH3C(O)OONO2) have been studied using a variety of theoretical methods. Reaction enthalpies calculated with the complete basis set (CBS) method were evaluated against a set of atmospheric free radical reactions for which the experimental thermochemistry is well defined. The validation procedure, which included two decomposition channels for HOONO2, demonstrated that the CBS-Q method reproduced the experimental Delta K-rxn(298K) values with a rms error of 5.7 kJ mol(-1). We report new Delta H-f(298K) values for PAN, CH3C(O)O-2, and CH3C(O)O of -240.1, -154.4 and -192.5 kJ mol(-1), respectively. Accurate structural calculations for PAN, HOONO2, CH3C(O)O-2, and CH3C(O)O augment the thermochemical calculations and show that the B3LYP density functional method describes the chemical bonding in the -OONO2 linkage quite well. The implications of this study for the atmospheric decomposition of PAN are discussed.