Reactions of laser-ablated Fe atoms and O-2 molecules in argon condensing on a 10 K surface have produced sharp infrared absorptions at 1204.5, 945.6, and 797.1 cm(-1) that are identified as FeOO and OFeO, respectively, through the use of isotopic substitution and DFT structure and isotopic frequency calculations. Annealing to 30 and 50 K produced a strong 956.0-cm(-1) band, which is due to cyclic Fe(O-2); visible photolysis destroyed the 1204.5 cm(-1) FeOO band, but it was regenerated on annealing. Formation of the more stable dioxo insertion product OFeO requires activation energy whereas the superoxo and peroxo addition products FeOO and Fe(O-2) can be formed on diffusion of cold reagents in solid argon. The FeOO spectrum observed and calculated here compares favorably with those reported for oxyhemoproteins, which provides further support for the Fe-O-O binding geometry in these biologically important metalloproteins.