Copper(l) complexes of sterically hindered anilido imine ligands o-C6H4{N((C6H3Pr2)-Pr-i)}{C(R)=(NC6H3Pr2)-Pr-i}(-) (L-1, R = H; L-2, R = CH3) have been prepared and characterized by spectroscopic and X-ray crystallographic methods. These complexes are highly reactive with O-2, and in the case of L-2 the product of low-temperature oxygenation was fully characterized by spectroscopic, X-ray crystallographic, and computational methods. The resonance Raman spectrum features an isotope-sensitive vibration at 974 cm(-1) (Delta(O-18) = 66 cm(-1)), consistent with assignment as an O-O stretch. Despite the asymmetric coordination environment provided by the supporting anilido imine ligand, the X-ray crystal structure confirms rather symmetric side-on binding of the O-2 moiety to the copper center, and the O-O bond length of 1.392(2) angstrom indicates that this intermediate has significant Cu(lll)-peroxo character. Theoretical calculations support this interpretation and predict that while a fully optimized end-on singlet geometry can be obtained, it is higher in energy than the side-on isomer by 3.5 kcal mol(-1) at the CASPT2/TZP level.