The electronic structure and thermochemical stability of the HOO-SO3 complex is studied using both second-order Moller-Plesset perturbation theory (MP2) and the B3LYP density-functional theory (DFT) method. The calculated dissociation energies of the complex are 10.25 and 11.51 kcal mol(-1) at the G3(MP2) and G3 levels, respectively. Anharmonic OH stretching frequencies of the HO2 moiety along with the frequency shifts upon complex formation are calculated at the MP2/6-311++G(2df,2p) and B3LYP/6-311++G(2df,2p) levels, and also AIM analyses of the MP2 and Kohn-Sham densities were performed. Theoretical data strongly encourage performing of matrix-isolation studies of the title complex and its spectroscopic identification. (C) 2004 Elsevier B.V. All rights reserved.