Spectroscopic properties of H2O2-HO2 have been re-investigated by post Hartree-Fock methods including anharmonic vibrational effects through a second order perturbative approach. The results have been compared to recent experimental data. The very low barrier height computed for the conversion of the five-membered complex into its more stable six-membered counterpart (27 cm(-1)) explains why only the six-membered hydrogen-bonded complex is observed in argon matrices. The calculated anharnionic frequencies of the latter complex are in excellent agreement with experimental data. (C) 2004 Published by Elsevier B.V.