The reactive oxidizing species in the selective oxidation of methane to methanol in oxygen activated Cu-ZSM-5 was recently defined to be a bent mono(mu-oxo)dicopper(II) species, [Cu2O](2+). In this communication we report the formation of an O-2-precursor of this reactive site with an associated absorption band at 29 000 cm(-1). Laser excitation into this absorption feature yields a resonance Raman (rR) spectrum characterized by O-18(2) isotope sensitive and insensitive vibrations, vO-O and vCu-Cu, at 736 (Delta O-18(2) = 41 cm(-1)) and 269 cm(-1), respectively. These define the precursor to be a mu-(n(2):n(2)) peroxo dicopper(II) species, [Cu-2(O-2)](2+). rR experiments in combination with UV-vis absorption data show that this [Cu-2(O-2)](2+) species transforms directly into the [Cu2O](2+) reactive site. Spectator Cu+ sites in the zeolite ion-exchange sites provide the two electrons required to break the peroxo bond in the precursor. O-2-TPD experiments with O-18(2) show the incorporation of the second O-18 atom into the zeolite lattice in the transformation of [Cu-2(O-2)](2+) into [Cu-2(O-2)](2+). This study defines the mechanism of oxo-active site formation in Cu-ZSM-5.