Employing a tetradentate N3S(thioether) ligand, L-N3S, dioxygen reactivity of a copper(I) complex, [(L-N3S) Cu-I](+) (1) was examined. In CH2Cl2, acetone (at -80 degrees C), or 2-methyltetrahydrofuran (at -128 degrees C), 1 reacts with O-2 producing the end-on bound peroxodicopper(II) complex [{(L-N3S)Cu-II}(2)(mu-1,2-O-2(2-))](2+) (2), the first reported copper dioxygen adduct with sulfur ( thioether) ligation. Its absorption spectrum contains an additional low-energy feature (but not a Cu-S CT band) compared to the previously well-characterized N-4 ligand complex, [{(TMPA)Cu-II}(2)(mu-1,2-O-2(2-))](2+) (3) (TMPA) = tris(2-pyridylmethyl) amine). Resonance Raman spectroscopy confirms the peroxo formulation {v((O-O)) = 817 cm(-1) (O-16-18(2) Delta = 46 cm(-)1) and v((Cu-O)) = 545 cm(-1) (O-16-18(2) Delta = 26 cm(-1)), in close analogy to that known for 3 {v((O-O)) = 827 cm(-1) and v((Cu-O)) = 561 cm(-1)}. Direct evidence for thioether ligation comes from EXAFS spectroscopy {Cu K-edge; Cu-S = 2.4 A}.