Reactions of selenium and oxygen were performed using five different methods, and the products were trapped for matrix infrared spectroscopic study. Based on different chemistry, photolysis and annealing behavior, selenium and oxygen isotopic substitution, and DFT structure and frequency calculations, several new selenium oxides are characterized. In addition to SeO, SeO2, and SeO3, and the (Se-2)(O-2) complex, the new molecules SeSeO, SeOO, Se2O2, and OSeOO are identified. Several isomers of Se2O2 are accidentally almost degenerate; the species identified, which contains a three-membered ring with an exocyclic Se=O unit, is one of the most stable. The OSeOO species is probably a planar cis chain isomer (C-s symmetry), which is calculated to lie about 100 kJ/mol above the D-3h structure of SeO3.