The cocondensation reaction of nickel atoms with oxygen or dilute oxygen–argon matrices at 4.2–10 °K gives rise to the first examples of binary transition metal dioxygen complexes. Variable concentration and diffusion controlled warm up studies and 16O2, 16O18O, 18O2 isotope substitution studies establish the stoichiometries of the dioxygen complexes of nickel to be Ni(O2) and (O2)Ni(O2). Both complexes have the dioxygen molecule(s) coordinated to the nickel atom in a "sideways" fashion.