Contrasted here are the competitive O-18/O-16 kinetic isotope effects ((OKIEs)-O-18) on Kcal/Km(O-2) for three non-heme iron enzymes that activate O-2 at an iron center coordinated by a 2-His-1-carboxylate facial triad: taurine dioxygenase (TauD),(S)-(2)-hydroxypylphosphonic acid epoxidase (HppE), and 1-aminocyclopropyl-1-carboxylic acid epoxidase (HppE), and 1-aminocyclopropyl-1-carboxylic acid oxidase (ACCO). Measured O-18 KIEs of 1.0102 +/- 0.0002 (TauD), 1.0120 +/-0.0002 (HppE), and 1.025 +/- 0.0005 (ACCO) suggest the formation in the rate-limiting step of O-2 activation of and FeIII-peroxohemiketal, FeIII-OOH, and FeIV=O species, respectively. The comparison of the measured O-18 KIEs with calculated or experimental O-18 equilibrium isotope effects (O-18 EIEs) provides new insights into the O-2 activatin through an inner-sphere mechanism at a non-heme iron center.