S-Oxygenation of thiophenolate bridges by ethereal hydroperoxides was studied. [NiII2LS(PhCO2)]+ (1), where LS = macrocyclic aminethiolate supporting ligand, is S-oxygenated readily in a mixed methanol/acetonitrile solution with ether/dioxygen at room temperature in the presence of daylight. The reactions were found to depend strongly on the choice of the ether. Uptake of two O atoms occurs in dioxane to give a mixed thiolate/sulfinate complex [NiII2LSO2(PhCO2)]+ (2) containing the rare five-membered Ni(μ1,1-S)(μ1,2-OS)Ni core. In tetrahydrofuran, four O atoms are taken up by 1 to generate the bis(sulfinate) species [NiII2LSO4(PhCO2)]+ (3). A mono-S-oxygenated sulfenate intermediate can be detected by electrospray ionization mass spectrometry. The oxygenation reactions proceed in high yields without complex disintegration and invariably provide μ1,2-bridging sulfinates as established by spectroscopy (IR and UV/vis), X-ray crystallography, and accompanying density functional theory calculations. The oxygenation of the S atoms has a strong impact on the electronic structures of the nickel complexes. The monosulfinate complex 2 has an S = 2 ground state resulting from moderate ferromagnetic exchange coupling interactions (J = +15.7 cm-1; H = -2JS1S2), while an antiferromagnetic exchange interaction in 3 shows the presence of a ground state with spin S = 0 (J = -0.56 cm-1).