The acetylperoxy + HO2 reaction has multiple impacts on the troposphere, with a triplet pathway leading to peracetic acid + O2 (reaction (1a)) competing with singlet pathways leading to acetic acid + O3 (reaction (1b)) and acetoxy + OH + O2 (reaction (1c)). A recent experimental study has reported branching fractions for these three pathways (α1a, α1b, and α1c) from 229 K to 294 K. We constructed a theoretical model for predicting α1a, α1b, and α1c 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-HO2 pre-reactive complexes led to a 298 K triplet rate constant, k1a = 5.11 × 10-12 cm3 per molecule per s, and values of α1a 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, k1b+1c = 1.20 × 10-11 cm3 per molecule per s, in good agreement with experiment. However, our predicted variations in α1b and α1c 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 + O3 formation vs. acetoxy + OH + O2 formation.