OH formation has been observed directly from the reaction between CH3CO and O-2. This reaction proceeds via the formation of energised CH3CO(O-2), which can either dissociate to OH or be collisionally stabilized to CH3CO(O-2). The kinetics of OH formation were measured as a function of pressure at 295 and 213 K, giving rate coefficients in good agreement with the literature. The OH yield was also determined as a function of pressure enabling the reaction to be further quantified. (C) 2002 Elsevier Science B.V. All rights reserved.