Abstract. This work details the first direct observation of OH as a product from (R1): HO 2 +CH 3 C(O)O 2 →(products), which has generally been considered an atmospheric radical termination process. The technique of pulsed laser photolysis radical generation, coupled to calibrated laser induced fluorescence detection was used to measure an OH product yield for (R1) of α 1 (298 K)=(0.5±0.2). This study of (R1) included the measurement of a rate coefficient k 1 (298 K)=(1.4±0.5)×10 −11 cm 3 molecule −1 s −1 , substantially reducing the uncertainties in modelling this important atmospheric reaction. OH was also detected as a product from the reactions of HO 2 with three other carbonylcontaining peroxy radicals, albeit at smaller yield, e.g. (R2): HO 2 +CH 3 C(O)CH 2 O 2 →(products), α 2 ≈0.15. By contrast, OH was not observed (α<0.06) as a major product from reactions where carbonyl functionality was absent, e.g. HO 2 +HOCH 2 CH 2 O 2 (R8), and HO 2 +CH 3 CH(OH)CH 2 O 2 (R9).