The thermochemical stability of the HO2-HClO4 complex is studied using both second-order Moller-Plesset perturbation theory (MP2) and the B3LYP density functional theory (DFT) method. Our calculations reveal stabilization energies of 11.1 and 13.2 kcal mol(-1) at the MP2/6-311++G(3df, 3pd) and B3LYP/6-311++G(3df, 3pd) levels, respectively. (C) 2003 Published by Elsevier B.V.