A manganese atom reacts with dioxygen to form the previously characterized MnO2 molecule in solid argon under UV-visible light irradiation. Subsequent sample annealing allows the dioxygen molecules to diffuse and to react with MnO2 to give the (eta(2)-O-2)MnO2 complex, which is characterized to be a side-on bonded peroxo manganese dioxide complex. The manganese tetraoxide MnO4, which was predicted to be less stable than the (eta(2)-O-2)MnO2 isomer, was not observed. However, the (eta(2)-O-2)MnO2 complex reacts with another weakly coordinated dioxygen to give the (eta(2)-O-2)MnO4 complex via visible light irradiation, in which the manganese tetraoxide is coordinated and stabilized by a side-on bonded O-2 molecule. Manganese dimer reacts with dioxygen to form the cyclic Mn(mu-O)(2)Mn cluster spontaneously upon annealing, which further reacts with dioxygen to give the (eta(2)-O-2)(2)Mn(mu-O)(2)Mn cluster, a side-on bonded disuperoxide complex with a planar D-2h structure.