We propose a non-radical mechanism for the conversion of methane into methanol by soluble methane monooxygenase (sMMO), the active site of which involves a diiron active center. We assume the active site of the MMOHQ intermediate, exhibiting direct reactivity with the methane substrate, to be a bis(μ-oxo)diiron(IV) complex in which one of the iron atoms is coordinatively unsaturated (five-coordinate). Is it reasonable for such a diiron complex to be formed in the catalytic reaction of sMMO? The answer to this important question is positive from the viewpoint of energetics in density functional theory (DFT) calculations. Our model thus has a vacant coordination site for substrate methane. If MMOHQ involves a coordinatively unsaturated iron atom at the active center, methane is effectively converted into methanol in the broken-symmetry singlet state by a non-radical mechanism; in the first step a methane CH bond is dissociated via a four-centered transition state (TS1) resulting in an important intermediate involving a hydroxo ligand and a methyl ligand, and in the second step the binding of the methyl ligand and the hydroxo ligand through a three-centered transition state (TS2) results in the formation of a methanol complex. This mechanism is essentially identical to that of the methane–methanol conversion by the bare FeO+ complex and relevant transition metal–oxo complexes in the gas phase. Neither radical species nor ionic species are involved in this mechanism. We look in detail at kinetic isotope effects (KIEs) for H atom abstraction from methane on the basis of transition state theory with Wigner tunneling corrections.