Contrasted here are the competitive 18O/16O kinetic isotope effects (18O KIEs) on kcat/Km(O2) for three non-heme iron enzymes that activate O2 at an iron center coordinated by a 2-His-1-carboxylate facial triad: taurine dioxygenase (TauD), (S)-(2)-hydroxypropylphosphonic acid epoxidase (HppE), and 1-aminocyclopropyl-1-carboxylic acid oxidase (ACCO). Measured 18O KIEs of 1.0102 +/- 0.0002 (TauD), 1.0120 +/- 0.0002 (HppE), and 1.0215 +/- 0.0005 (ACCO) suggest the formation in the rate-limiting step of O2 activation of an FeIII-peroxohemiketal, FeIII-OOH, and FeIV O species, respectively. The comparison of the measured 18O KIEs with calculated or experimental 18O equilibrium isotope effects (18O EIEs) provides new insights into the O2 activation through an inner-sphere mechanism at a non-heme iron center.