Electrospray mass spectrometry is used to investigate the reactivity of the coordinatively unsaturated ions [M(bipy)(2)](2+) (M = Cr, Ru, Os) and [M(bipy)](+) (M = Co, Ni) toward O-2 to give the dioxygen adducts [M(bipy)(2)O-2](2+) (M = Cr, Ru, Os) and [M(bipy)O-2](+) (M = Co, Ni). CID experiments with the [M(bipy)(2)O-2](2+) and [M(bipy)O-2](+) adducts show that the loss of a mass equivalent to two oxygen atoms is the predominant process for [M(bipy)(2)O-2](2+) (M = Cr, Ru) and [M(bipy)O-2](+) (M = Ni, Co). In contrast, weak signals assigned to [Os(bipy)O-2](+), [Os(bipy)O-2](2+), and bipy(+.) indicate loss of a bipyridine ligand from [Os(bipy)(2)O-2](2+) rather than O-2 loss. This result may have structural implications and a dioxo Os(VI) complex cation, [Os(bipy)(2)(O)(2)](2+), is proposed for the Os-dioxygen adduct. This assignment is consistent with the known solution and solid-state chemistry of osmium. Formation of [Os-VI(bipy)(2)(O)(2)](2+) involves a formal oxidation of the Os(II) to an Os(VI) and concomitant O-2 four-electron reduction of O-2. Even though loss of O-2 rather than bipy was the predominant process observed in CID experiments for the Ru adduct, a structural formulation of an analogous dioxo species is not excluded for [Ru(bipy)(2)O-2](2+).