For the isomerization of HOONO to nitric acid, a spin triplet reactive intermediate (HOONO)-H-3* was identified on a two state reactivity (TSR) potential energy surface. This bi-radical is postulated as one of the possible activated species responsible for the potent oxidant activity attributed to H-peroxynitrite. A theoretical analysis based on spin density-dependent reactivity indices consistently explains the observed reactivity of this molecule in biological and model systems. (C) 2008 Elsevier B.V. All rights reserved.