A theoretical estimation of the enthalpy of formation for the hydroperoxyl radical is presented. These results are based on CCSD(T)/aug-cc-pCV5Z calculations extrapolated to the basis-set limit with additional corrections. Anharmonic vibrational zero-point energies, scalar relativistic, spin-orbit coupling, and diagonal Born-Oppenheimer corrections are further used to correct the extrapolated term energies, as well as various empirical corrections that account for correlation effects not treated at the CCSD(T) level. We estimate that Delta(f)H(0)degrees = 3.66 +/- 0.10 kcal mol(-1) (Delta(f)H(298)degrees = 2.96 +/- 0.10 kcal mol(-1)) using several reaction schemes. Significantly, it appears to be necessary to include effects of connected pentuple excitations in order to achieve an uncertainty of ca. 0.1 kcal mol(-1).