[(ANS)CuI(CH3CN)]+ reacts with O2 giving [{(ANS)CuII}2(μ-η2:η2-O22−)]2+, νO–O = 731 cm−1, shown to possess S-thioether ligation, based on comparisons with analogues having all N-ligands or a –S(Ph) group. The finding is a rare occurrence and new for side-on O22− binding.