In this work, the structure, the vibrational spectrum and binding energy of HO2 . H2O2 radical complexes are predicted for the first time using ab initio molecular orbital methods. Two equilibrium structures of the HO2 . H2O2 radical complexes have been determined, structure 1 is a six-membered ring with two distorted hydrogen bonds, structure 2 is a five-membered ring with the other atoms in the H2O2 out of the plane. Theory predicts that the binding energy D-0 of structures 1 and 2 is 5.3 and 4.67 kcal/mol, respectively, using CCSD(T) method. (C) 2002 Elsevier Science B.V. All rights reserved.