Cryoreduction of the [FeO2](6) (n = 6 is the number of electrons in 3d orbitals on Fe and pi* orbitals on O-2) dioxygen-bound ferroheme through gamma irradiation at 77 K generates an [FeO2](7) reduced oxy-heme. Numerous investigations have examined [FeO2](7) centers that have been characterized as peroxo-ferric centers, denoted [FeO2](per)(7), in which a ferriheme binds a dianionic peroxo-ligand. The generation of such an intermediate can be understood heuristically if the [FeO2](6) parent is viewed as a superoxo-ferric center and the injected electron localizes on the O-O moiety. We here report EPR/ENDOR experiments which show quite different properties for the [FeO2](7) centers produced by cryoreduction of monomeric oxy-hemoglobin (oxy-GMH3) from Glycera dibranchiata, which is unlike mammalian "globins" in having a leucine in place of the distal histidine; of frozen aprotic solutions of oxy-ferrous octaethyl porphyrin; and of the oxy-ferrous complex of the heme model, cyclidene. These [FeO2](7) centers are characterized as "superoxo-ferrous" centers ([FeO2](sup)(7)), With nearly unit spin density localized on a superoxo moiety which is end-on coordinated to a low-spin ferrous ion. This assignment is based on their g tensors and O-17 hyperfine couplings, which are characteristic of the superoxide ion coordinated to a diamagnetic metal ion, and on the absence of detectable ENDOR signals either from the in-plane N-14 ligands or from an exchangeable H-bond proton. Such a center would arise if the electron that adds to the [FeO2](6) superoxo-ferric parent localizes on the Fe ion, to make a superoxo-ferrous moiety. Upon annealing to T > 150 K, the [FeO2](sup)(7) species converts to peroxo/hydroperoxo-ferric ([FeO2H](7)) intermediates. These experiments suggest that the primary reduction product is [FeO2](sup)(7) and that the internal redox transition to [FeO2](per)(7)/[FeO2H](7) states is driven at least in part by H-bonding/proton donation by the environment.