The reaction between C2H5CO and O-2 (R1) is observed to directly form OR This reaction proceeds via the formation of energised C2H5CO(O-2), which can either dissociate to OH or be collisionally thermalized. This reaction mechanism is analogous to CH3CO + O-2 (R4), which we have previously studied and extended in this study. The kinetics and OH yield of reactions R1 and R4 are reported as a function of pressure at 295 and 213 K, and are sufficiently different to allow the identification of the corresponding acyl radical and it is demonstrated for 2-butanone photolysis. (c) 2005 Elsevier B.V. All rights reserved.