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The acetylperoxy + HO 2 reaction has multiple impacts on the troposphere, with a triplet pathway leading to peracetic acid + O 2 (reaction (1a)) competing with singlet pathways leading to acetic acid + O 3 (reaction (1b)) and acetoxy + OH + O 2 (reaction (1c)). A recent experimental study has reported branching fractions for these three pathways ( α 1a , α 1b , and α 1c ) from 229 K to 294 K. We constructed a theoretical model for predicting α 1a , α 1b , and α 1c using quantum chemical and Rice–Ramsperger–Kassel–Marcus/master equation (RRKM/ME) simulations. Our main quantum chemical method was Weizmann-1 Brueckner Doubles (W1BD) theory; we combined W1BD and equation-of-motion spin-flip coupled cluster (SF) theory to treat open-shell singlet structures. Using RRKM/ME simulations that included all conformers of acetylperoxy–HO 2 pre-reactive complexes led to a 298 K triplet rate constant, k 1a = 5.11 × 10 −12 cm 3 per molecule per s, and values of α 1a in excellent agreement with experiment. Increasing the energies of all singlet structures by 0.9 kcal mol −1 led to a combined singlet rate constant, k 1b+1c = 1.20 × 10 −11 cm 3 per molecule per s, in good agreement with experiment. However, our predicted variations in α 1b and α 1c with temperature are not nearly as large as those measured, perhaps due to the inadequacy of SF theory in treating the transition structures controlling acetic acid + O 3 formation vs. acetoxy + OH + O 2 formation.