The epoxidation of ethene by a model for Compound I of cytochrome P450, studied by the use of density functional B3LYP calculations, involves two-state reactivity (TSR) with multiple electromer species, hence "multi-state epoxidation". The reaction is found to proceed in stepwise and effectively concerted manners. Several reactive states are involved; the reactant is an (oxo)iron(IV) porphyrin cation radical complex with two closely lying spin states (quartet and doublet), both of which react with ethene to form intermediate complexes with a covalent C-O bond and a carbon-centered radical (radical intermediates). The radical intermediates exist in two electromers that differ in the oxidation state of iron; Por(+). (FeOCH2CH2)-O-III. and PorFe(IV)-OCH2CH2. (Por = porphyrin). These radical intermediates exist in both the doublet- and quartet spin stares. The quartet spin intermediates have substantial barriers for transformation to the quartet spin PorFe(III)-epoxide complex (2.3 kcal mol(-1) for PorFe(IV)OCH(2)CH(2). and 7.2 kcal mol(-1) for Por(+). (FeOCH2CH2)-O-III.). In contrast, the doublet spin radicals collapse to the corresponding PorFe(III)-epoxide complex with virtually no barriers. Consequently, the lifetimes of the radical intermediates are much longer on the quartet- than on the doublet spin surface. The loss of isomeric identity in the epoxide and rearrangements to other products arise therefore mostly, if not only, from the quartet process, while the doublet state epoxidation is effectively concerted (Scheme 7). Experimental trends are discussed in the light of the computed mechanistic scheme, and a comparison is made with closely related mechanistic schemes deduced from experiment.