Abstract The rate constants k d for the unimolecular decomposition of two acyl (RĊO) radi‐ cals: 2,2‐dimethylpropionyl (CH 3 ) 3 CĊO and 2‐methylpropionyl (CH 3 ) 2 CHĊO (pivaloyl and isobutyryl, respectively in what follows) have been directly measured via the time‐resolved kinetics of CO formation as a function of pressure and temperature. The acyl radicals are generated by H‐atom abstraction of the aldehydic hydrogen from the corresponding substituted propanals using pulsed laser photolysis of aldehyde/Cl 2 /N 2 mixtures. The buildup of CO concentration is monitored by time‐resolved infrared absorption at ≅2177 cm ‐1 . The extent of the chain reactions triggered by the reactions of Cl 2 with alkyl and acyl radicals (thus generating further Cl atoms after the initial laser pulse) has been limited by adding variable known concentrations of O 2 , a radical scavenger which is sequestering all radicals to form less reactive RO 2 radicals. Besides k d , we have also determined for the first time the rate constants of the reactions of the two acyl radicals with O 2 ; since the latter do not exhibit any clear trend with either temperature or pressure, we thus adopt the average of all experimental values: i.e. for pivaloyl : k = (3.5 ± 1.0) × 10 −12 cm 3 molecule −1 s −1 , and for isobutyryl : k = (3.2 ± 1.0) × 10 −12 cm 3 molecule −1 s −1 . Our values of k d , which are in the falloff range, are compared to those obtained from theoretical calculations using the Troe formalism 5 . For pivaloyl at 298 K, we obtain the following limiting rate constants : k 0 = 2.1 × 10 −13 cm 3 molecule −1 s −1 , k ∞ = 4.3 × 10 5 s −1 , F C = 0.515, which are at variance with the theoretical predictions of Tomas et al. (Phys Chem Chem Phys 2000, 2, 1165–1174). For isobutyryl, our experimental data are entirely consistent with the predictions of Tomas et al. Extrapolation of our Troe parameters to tropospheric conditions (298 K, atmospheric pressure) shows, in agreement with previous authors, that for both radicals the reaction with O 2 is several orders of magnitude faster than the unimolecular decomposition. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 611–624, 2005