The hydrotrioxyl radical (HOOO(•)) plays a crucial role in atmospheric processes involving the hydroxyl radical (HO(•)) and molecular oxygen (O2). The equilibrium geometry of the electronic ground state (X (2)A'') of the trans conformer of HOOO(•) and its unimolecular dissociation into HO(•) (X (2)Π) and O2 (X (3)Σg(-)) have been studied theoretically using CASSCF and CASPT2 methodologies with the aug-cc-pVTZ basis set. On the one hand, CASSCF(19,15) calculations predict for trans-HOOO(•) (X (2)A'') an equilibrium structure showing a central O-O bond length of 1.674 Å and give a classical dissociation energy De = 1.1 kcal/mol. At this level of theory, it is found that the dissociation proceeds through a transition structure involving a low energy barrier of 1.5 kcal/mol. On the other hand, CASPT2(19,15) calculations predict for trans-HOOO(•) (X (2)A'') a central O-O bond length of 1.682 Å, which is in excellent agreement with the experimental value of 1.688 Å, and give De = 5.8 kcal/mol. Inclusion of the zero-point energy correction (determined from CASSCF(19,15)/aug-cc-pVTZ harmonic vibrational frequencies) in this De leads to a dissociation energy at 0 K of D0 = 3.0 kcal/mol. This value of D0 is in excellent agreement with the recent experimentally determined D0 = 2.9 ± 0.1 kcal/mol of Le Picard et al. (Science 2010, 328, 1258-1262). At the CASPT2 level of theory, we did not find for the dissociation of trans-HOOO(•) (X (2)A'') an energetic barrier other than that imposed by the endoergicity of the reaction. This prediction is in accordance with the experimental findings of Le Picard et al., indicating that the reaction of HO(•) with O2 yielding HOOO(•) is a barrierless association process.