The data in Table shows that O isotope fractionation and thus O-ϵ varies substantially from no fractionation (0.1 ± 0.7) to -30 for all probe compounds studied. As a general trend, O isotope fractionation increases (i.e., O-ϵ values become more negative) with decreasing initial probe compound concentration. The extent of O isotope fractionation, however, did not correlate with reaction order. An interpretation of the observed trends of O-ϵ values follows from the evaluation of isotope effects pertinent to the elementary reaction steps of O 2 activation to 1 O 2 and its reaction with the probe compounds outlined in eq 3.