Data for 18 O-AKIEs for furfuryl alcohol, unfortunately, only extent do k 3 [PC]/k 2 of 0.12 and thus do not allow for observation of the maximum O isotope fractionation. Moreover, some O-AKIEs from reactions with histidine and methionine are approximately 0.002 to 0.003 AKIE units higher than rationalized with eq 10 for k 3 [PC]/k 2 values between 0.3 and 10. We hypothesize that the observed deviations from trends rationalized by eq 10 have two possible origins; distinct reaction mechanisms of 1 O 2 attack at the probe compounds and, possibly, different concentrations of 1 O 2 across the various experiments.