Cytochrome P450 monooxygenase and superoxide reductase (SOR) have the same first atom coordination shell at their iron active sites: an Fe[N4S] center in a square-pyramidal geometry with the sixth coordinate site open for the catalytic reaction. Furthermore, both pass through ferric hydroperoxo intermediates. Despite these similarities, the next step in their catalytic cycle is very different: distal oxygen protonation and O-O cleavage (P450) versus proximal oxygen protonation and H2O2 release (SOR). One of the factors leading to this difference is the spin state of the intermediates. Density functional theory (DFT) applied to models for the ferric hydroperoxo, (SCH3)(L)Fe-III-OOH (L = porphyrin for P450 and four imidazoles for SOR), gives different ground spin states; the P450 model with the porphyrin, which contains the Fe-N distances, prefers a low-spin ground state, whereas the SOR model with four histidines, in which Fe-N bonds are extendable, prefers a high-spin ground state. Their ground spin states lead to geometric and electronic structures that assist in (1) the protonation on distal oxygen for P450, which leads to O-O bond cleavage and formation of the oxo-ferryl, (SCH3)(L)Fe-IV=O (Cpd I), and H2O, and (2) the protonation on proximal oxygen for SOR, which leads to the formation of the ferric hydrogen peroxide, (SCH3)(L) Fe-III-HOOH, intermediate before the Fe-O bond cleavage and H2O2 production. Specifically, the quartet ground state of the water-bound oxo-ferryl, (SCH3)(L)Fe-IV=O center dot center dot center dot H2O, is more stable than the sextet ground state of (SCH3)(L)Fe-III-HOOH by -14.29 kcal/mol for the P450 model. Another important factor is the differences in the location of the active site: P450's active site is embedded within the enzyme, whereas SOR's active site is exposed to the aqueous environment. In the latter location, water molecules can freely form hydrogen bonds with both proximal and distal oxygen to stabilize the (SCH3)(L)Fe-III-HOOH intermediate. When two explicit water molecules are included in the model, the sextet ground state of (SCH3)(L)Fe-III-HOOH center dot center dot center dot 2H(2)O is more stable than the quartet ground state of (SCH3)(L)Fe-IV=O center dot center dot center dot 3H(2)O by -2.14 kcal/mol for the SOR model. Our calculations show that both the spin state, which is controlled by the differences between four N donors in porphyrin versus those in imidazoles, and the degree of solvent exposure of the active sites play important roles in the fate of the (SCH3)(L)Fe-III-OOH intermediate, leading to O-O cleavage in one situation (P450) and hydrogen peroxide production in the other (SOR).