The ferrous square-pyramidal [Fe(NHis)(4)(SCYs)] site of superoxide reductases (SORs) has been shown to reduce superoxide at a nearly diffusion-controlled rate. The final products of the reaction are hydrogen peroxide and the ferric hexacoordinated SOR site, with a carboxylate group from a conserved glutamate serving as the sixth ligand trans to the cysteine sulfur. A transient intermediate absorbing at similar to600 nm in the reaction of the ferrous pentacoordinated site with superoxide has been proposed to be a ferric-(hydro)peroxo complex (Coulter, E.; Emerson, J.; Kurtz, D. M., Jr.; Cabelli, D. J. Am. Chem. Soc. 2000, 122, 11555-11556.). In the present study, DFT and ZINDO/S-Cl results are shown to support the description of the 600-nm intermediate as an end-on, low-spin ferric-peroxo or -hydroperoxo complex. Side-on peroxo coordination was found to be significantly less stable than end- on because of constraints on the imidazole ligand ring orientations imposed mostly by the protein. The modeled ferric-hydroperoxo complex had a decidedly nonplanar CysCbeta-S-Fe-O-O geometry that appears to be imposed by the same constraints. A single prominent visible absorption of the (hydro)peroxo model is shown to be due mainly to a CysS --> Fe(ill) pi charge transfer (CT) transition with a minor portion of His --> Fe(III) pi CT character and very little peroxo --> Fe(III) CT character. On the basis of calculations of models with various mono- and diprotonated peroxo ligands, protonation of the iron-bound peroxo oxygen is a key step in the decay of the ferric(hydro)peroxo complex favoring release of hydrogen peroxide over cleavage of the O-O bond, as occurs in the heme structural analogue, cytochrome P450.