A kinetic study of the reaction between a diiron(II) complex [FeII2(μ-OH)2(6-Me3-TPA) 2]2+ 1, where 6-Me3-TPA = tris(6-methyl-2-pyridylmethyl)amine, and dioxygen is presented. A diiron(III) peroxo complex [FeIII2(μ-O)-(μ-O2) (6-Me3-TPA)2]2+ 2 forms quantitatively in dichloromethane at temperatures from -80 to -40 °C. The reaction is first order in [FeII2] and [O2], with the activation parameters ΔH‡ = 17 ± 2 kJ mol-1 and ΔS‡ = -175 ± 20 J mol-1 K-1. The reaction rate is not significantly influenced by the addition of H2O or D2O. The reaction proceeds faster in more polar solvents (acetone and acetonitrile), but the yield of 2 is not quantitative in these solvents. Complex 1 reacts with NO at a rate about 103 faster than with O2. The mechanistic analysis suggests an associative rate-limiting step for the oxygenation of 1, similar to that for stearoyl-ACP Δ9-desaturase, but distinct from the probable dissociative pathway of methane monoxygenase. An η1-superoxo FeIIFeIII species is a likely steady-state intermediate during the oxygenation of complex 1.