Enthalpies of formation of the HOOOH molecule and HOOO radical were determined accurately employing density functional (DFT), coupled-clusters (CC) and complete basis set (CBS) extrapolation methods. The enthalpy of formation of the HO3 radical was determined as 7.1 +/- 2 kcal/mol at 298 K through a series of calculations employing the isodesmic reaction HOOOH + OH - HOH + HOOO. This value is in disagreement with the experimental one of Speranza, -1 +/- 5 kcal/mol. The enthalpy of formation of HOOOH, calculated at the extrapolated CBS/CCSD(T) and CBS-APNO levels, amounts to -21.1 +/- 1 kcal/mol, also in disagreement with the experimentally determined upper limit of -26.0 kcal/mol. Further examination of the procedure used to determine this value from the raw experimental data, suggests that this value is not as reliable as originally thought (by Speranza). The data should be reexamined and perhaps supplemented with additional experiments. (C) 2002 Published by Elsevier Science B.V.