The reaction between C 2 H 5 CO and O 2 (R1) is observed to directly form OH. This reaction proceeds via the formation of energised C 2 H 5 CO(O 2 ), which can either dissociate to OH or be collisionally thermalized. This reaction mechanism is analogous to CH 3 CO+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.