The potential energy surface for the HO3 --> HO + O-2 reaction has been studied using the QCISD(T)/CBS (complete basis set) method. The HO3 molecule is found to be stable relative to the HO + O-2 dissociation limit. It has a planar equilibrium geometry, with the cis-HO3 structure being slightly more stable than the trans-HO3 one. The classical dissociation barrier height is predicted to be 8.87 kcal mol(-1). Based on the calculated energy points, a new double many-body expansion (DMBE II) potential energy surface for HO3 was obtained.