The one-electron reduced iron(II)-dioxygen adduct, {Fe-II-O-2}(-), is known to be an important intermediate in the catalytic cycle of heme (mono)oxygenases. The same type of species, considered as Fe-III-peroxo, can be formed in a direct reaction between a Fe-II center and superoxide. In a unique high-pressure study of the reaction between superoxide and the Fe-II complex of a crown ether porphyrin conjugate in dimethylsulfoxide (DMSO), the overall Fe-II-superoxide interaction mechanism could be visualized and the nature of all species that occur along the reaction coordinate could be clarified. The equilibrium between the low-spin and high-spin forms of the starting Fe-II complex was quantified, which turns out to be the actual activation step toward substitution and subsequent inner-sphere electron transfer reactions. The constructed reaction volume profile demonstrates that the reaction product consists of Fe-II-peroxo and Fe-II-superoxo species that exist in equilibrium, which can better account for the versatile reactivity of {Fe-II-O-2}(-) adducts toward different substrates.