The hydroperoxyl radical, HO₂, was identified by Parise et al. toward the SM1 core of ρ Oph A in 2012 using the IRAM 30 m telescope as well as the Atacama Pathfinder EXperiment (APEX) telescope. Five rotational lines from 130 to 260 GHz were identified. The hydroperoxyl radical is thought to be formed on grain mantles and to be relevant to the formation of water and hydrogen peroxide in the interstellar medium.

The HO₂ radical forms when one H is removed from hydrogen peroxide. It posseses two low-lying electronic states: the X²A" ground state, where the singly occupied p-like orbital on oxygen is out of the molecular plane, and the A²A' excited state, where the singly occupied orbital is in the molecular plane. Bonding diagrams for the two states are shown below (the singly occupied orbitals are yellow).

HO2(X2A")	HO2(A2A')