Current interest in copper/dioxygen reactivity includes the influence of thioether sulfur ligation, as it concerns the formation, structures, and properties of derived copper-dioxygen complexes. Here, we report on the chemistry of {L-Cu-I}(2)-(O-2) species L = (ESE)-E-DMM, (ESP)-E-DMM, and (ESDP)-E-DMM, which are N3S(thioether)-based ligands varied in the nature of a substituent on the S atom, along with a related N3O(ether) (EOE) ligand. Cu-I and Cu-II complexes have been synthesized and crystallographically characterized. Copper(I) complexes are dimeric in the solid state, [{L-Cu-I}(2)](B(C6F5)(4))(2), however are shown by diffusion-ordered NMR spectroscopy to be mononuclear in solution. Copper(II) complexes with a general formulation [L-Cu-II(X)](n+) {X = ClO4-, n = 1, or X = H2O, n = 2} exhibit distorted square pyramidal coordination geometries and progressively weaker axial thioether ligation across the series. Oxygenation (-130 degrees C) of {((ESE)-E-DMM)Cu-I}(+) results in the formation of a trans-mu-1,2-peroxodicopper(II) species [{((ESE)-E-DMM)Cu-II}(2)(mu-1,2-O-2(2-))](2+) (1(P)). Weakening the Cu-S bond via a change to the thioether donor found in (ESP)-E-DMM leads to the initial formation of [{((ESP)-E-DMM)Cu-II}(2)(mu-1,2-O-2(2-))](2+) (2(P)) that subsequently isomerizes to a bis-mu-oxodicopper(III) complex, [{((ESP)-E-DMM)Cu-III}(2)(mu-O2-)(2)](2+) (2(O)), with 2(P) and 2(O) in equilibrium (K-eq = [2(O)]/[2(P)] = 2.6 at -130 degrees C). Formulations for these Cu/O-2 adducts were confirmed by resonance Raman (rR) spectroscopy. This solution mixture is sensitive to the addition of methylsulfonate, which shifts the equilibrium toward the bis-mu-oxo isomer. Further weakening of the Cu-S bond in (ESDP)-E-DMM or substitution with an ether donor in (EOE)-E-DMM leads to only a bis-mu-oxo species (3(O) and 4(O), respectively). Reactivity studies indicate that the bis-mu-oxodicopper(III) species (2(O), 3(O)) and not the trans-peroxo isomers (1(P) and 2(P)) are responsible for the observed ligand sulfoxidation. Our findings concerning the existence of the 2(P)/2(O) equilibrium contrast with previously established ligand-Cu-I/O-2 reactivity and possible implications are discussed.