Copper ions exchanged onto Al-substituted zeolites facilitate various oxidation reactions and redox cycles, including NO decomposition, 1-2 CO oxidation, NOx (x = 1,2) selective catalytic reduction using NH3, and the partial oxidation of CH4 to form CH3OH in stoichiometric and catalytic cycles. In the context of partial methane oxidation (PMO), Cu(II) ions reduce to their Cu(I) states at high temperatures (e.g., 723 K in He, 523 K in 5% CO ) prior to exposure to extended O2 activation treatments (e.g., 723 K, > 2 h) that form active multinuclear Cu(II) species (CuxOy). At lower temperatures, these CuxOy intermediates react readily with CH4 to form surface methoxy-derived species (e.g., complete consumption within 0.5 h at 473 K), which desorb as CH3OH upon contact with H2O in vapor or liquid phases. The dependence of methanol yields (per Cu) on reaction conditions (e.g., reactant pressure, temperature, time) has been well studied; however, comparatively few investigations describe the series of steps responsible for the activation of O2 molecules or the kinetics and formation of active CuxOy complexes. Nearly two decades of research have sought to identify the CuxOy species that form upon O2 activation treatments of Cu-exchanged zeolites and produced numerous proposals (e.g., bis(μ-oxo)dicopper, mono(μ-oxo)dicopper, μ-(η 2 :η 2 )peroxo dicopper, trans-η-1,2peroxo dicopper, bis(μ-hydroxyl)dicopper , mono(μoxo)tricopper ). Initial reports from Groothaert et al. combined in situ extended X-ray absorption fine structure (EXAFS), UV-Vis-near-IR, and electron paramagnetic resonance (EPR) spectroscopies with comparisons to dicopper metalloenzymes to assign a 22,700 cm -1 UV-Vis absorbance feature observed on O2-activated Cu-ZSM-5 to a bis(μ-oxo)dicopper species. Solomon and coworkers utilized resonance Raman spectroscopy and DFT from which they concluded a bent mono-(μ-oxo)dicopper forms upon O2 activation over Cu-ZSM-5 and Cu-MOR. Recently, van Bokhoven and coworkers examined the formation of PMO active sites in MOR and MAZ zeolites and proposed that proximal [CuOH] + species form bridged Cu-oxo type species using X-ray powder diffraction, DFT, and XAS . Additionally, Pappas et al. studied Cu-MOR and Cu-CHA zeolites with varying Cu and Al content and found that O2 activation requires two Cu centers (i.e., to form mono(μ-oxo)dicopper or trans-η-1,2-peroxo dicopper) by using in situ X-ray absorption (XAS) and resonance Raman spectroscopies. Lobo and coworkers studied the small-pore zeolite Cu-CHA using Raman spectroscopy to identify vibrational features for trans-μ-1,2-peroxo dicopper and mono-(μ-oxo)dicopper, identifying the vibrational features of the latter using O-atoms and DFT calculations. In this study, mono-(μ-oxo)dicopper was determined to be the most stable CuxOy site formed as Raman spectra of the O2-activated sample remained unchanged after four days in O2 at ambient temperature. In contrast, Lercher and coworkers implicated mono(μ-oxo)tricopper intermediates in both Cu-ZSM-5 and Cu-MOR based on Cu-normalized stoichiometric CH3OH yields (O2 activation: pure O2, 723 K, 1 h; CH4 introduction: 91.2 kPa CH4 balance He, 473 K, 4 h; CH3OH extraction: equimolar H2O and He, 408 K, 0.5 h) that correlated linearly with one-third of the total Cu content, consistent with structures identified within in situ EXAFS spectra during O2 activation. Taken together, this series of contributions presents compelling evidence for multiple and seemingly contradictory conclusions regarding the identity of CuxOy species formed upon O2 activation over Cu-zeolites, which remains an important area for continued investigation.