The formation of bimolecular complexes of HOOOH and the HOOOﰀ radical with sulfuric acid (H2SO4– HOOOH, H2SO4–HOOOﰀ) has been investigated by using DFT (B3LYP/6-311++G(3df,3pd)) and CCSD(T)- F12 methods. For the first time the structures and the binding energies (BEs) for the various isomeric hydrogen-bonded complexes have been reported. The results reveal an unusually stable H2SO4–HOOOH eight-membered ring structure with two relatively strong hydrogen bonds, and with a calculated BE (CCSD(T)-F12) of 12.7 kcal molﰁ1. This is equivalent to the BE in the H2SO4–HOOH complex investigated at the same theoretical level. The complexation of HOOOﰀ with H2SO4 stabilizes this metastable polyoxide intermediate by about 10.0 kcal molﰁ1.