The identity of the species observed during O2 activation by Raman spectroscopy was evinced through a combination of isotopic labeling and computed Raman spectra of plausible chemical species. Isotopic labeling experiments with 18 O2 were performed by treating Cu-CHA-I and Cu-CHA-M at 723 K in flowing 18 O2 (21 kPa 18 O2, 80 kPa He) to identify vibrational shifts to aid in the identification of reactive CuxOy species formed. Both Cu-CHA-I and Cu-CHA-M possess Raman scattering features at 603 cm -1 when either 18 O2 or O2 is used (Figure ). Cu-CHA-M activated in O2 under dry conditions, however, possesses an additional feature around 580 cm -1 previously attributed to trans-μ-1,2peroxo dicopper(II). Consequently, these data suggest that the presence of H2O deliberately added to O2 or present in trace amounts within O2 aid in the structural conversion of trans-μ-1,2-peroxo dicopper(II) to form the species that exhibits a Raman scattering at 603 cm -1 . Notably, these findings differ from those of Lobo and coworkers, who observed an isotopic shift (Δ 18 O2) of 24 cm -1 for the vibration at 603 cm -1 . In their work, a scattering feature at 836 cm -1 was assigned to ν(O-O) of trans-μ-1,2-peroxo dicopper. Here, the lack of shift in the Raman spectra of 18 O2activated Cu-CHA zeolites seems to exclude this possibility. Consequently, the broad features centered around 800 and 1100 cm -1 (Figure ) appear to reflect framework νs(Si-O) and νas(Si-O) modes that are strongly scattered based upon preferential excitation (i.e., resonance) of CuxOy species they stabilize.