UMP2/aug-cc-pvdz calculations are used to analyze the interaction between hydroperoxyl radical (HOO) and CH3X (X = F, Cl, Br). Two minima are located on the potential energy surface of each complex. The more strongly bound contains a OH···X bond, along with CH···O; only CH···O bonds occur in the less stable minimum. Binding energies of the dominant minimum lie in the range of 20−24 kJ/mol, with X = F the most strongly bound. Analysis of the perturbations of the covalent bond lengths within each subunit caused by complexation, coupled with vibrational frequencies and charge transfers, opens a window into the nature of the interactions.