The purpose of this study is to give reliable and accurate thermochemical data for HO2, HO2+, and HO2. Their heats of formation were determined using quantum chemical calculations with the aid of high-accuracy coupled-cluster methods taking account of zero-point vibrational energies, scalar-relativistic effects, and the deficiencies of the Born-Oppenheimer approximation. Furthermore, a thermochemical network, containing 14 experimental and 7 theoretical reaction enthalpies, was set up to determine even more accurate heats of formation. The iteratively reweighted least-squares solution of the network yielded the best heat of formation estimates, which are Delta H-f(0)degrees(HO2) = 14.85 +/- 0.22, Delta H-f(298)degrees(HO2) = 11.92 +/- 0.22, Delta H-f(0)degrees(HO2+) = 1110.56 +/- 0.40, Delta H-f(298)degrees(HO2+) = 1107.64 +/- 0.40, Delta H-f(0)degrees(HO2) = -89.04 +/- 0. 39, and Delta H-f(298)degrees(HO2) = -91.75 +/- 0.39 kJ/mol. In addition, in line with previous accurate data Delta H-f(0)degrees(OH) = 37.25 +/- 0.03, Delta H-f(0)degrees(OH+) = 1293.20 +/- 0.03, and Delta H-f(0)degrees(H2O2) = -129.48 +/- 0.06 kJ/mol were also delivered by our network.