Bimolecular reactions in Earth's atmosphere are generally assumed to proceed between reactants whose internal quantum states are fully thermally relaxed. Here, we highlight a dramatic role for vibrationally excited bimolecular reactants in the oxidation of acetylene. The reaction proceeds by preliminary adduct formation between the alkyne and OH radical, with subsequent O-2 addition. Using a detailed theoretical model, we show that the product-branching ratio is determined by the excited vibrational quantum-state distribution of the adduct at the moment it reacts with O-2. Experimentally, we found that under the simulated atmospheric conditions O-2 intercepts similar to 25% of the excited adducts before their vibrational quantum states have fully relaxed. Analogous interception of excited-state radicals by O-2 is likely common to a range of atmospheric reactions that proceed through peroxy complexes.