Peroxynitrates are thermally unstable intermediates (at ambient temperatures) in the atmospheric degradation of hydrocarbons. In this work, thermal lifetimes of nine peroxynitrates have been measured as a function of temperature and, for two of them, also, as a function of total pressure. In the presence of excess NO, relative concentrations of the peroxynitrates were followed in a 420 I reaction chamber as a function of time by means of longpath IR absorption using a Fourier transform spectrometer. Original data on the unimolecular decomposition rate constants are presented for the peroxynitrates RO 2 NO 2 with R ϭ C 6 H 11 , CH 3 C(O)CH 2 , C 6 H 5 CH 2 , CH 2 I, CH 3 C(O)OC(H)CH 3 , C 6 H 5 OCH 2 , (CH 3 ) 2 NC(O), C 6 H 5 OC(O), and C 2 H 5 C(O). Thermal lifetimes at room temperature and atmospheric pressure are very short (in the order of seconds) for substituted methyl peroxynitrates (i.e., RЈCH 2 O 2 NO 2 ) but rather long for substituted formyl peroxynitrates (i.e., RЉC(O)O 2 NO 2 ). Kinetic data from this and previous work from our laboratory are used to derive structure-stability relationships which allow an estimate of the thermal lifetimes of peroxynitrates from readily available 13 C n.m.r. shift data.