The photoinduced dissociation of a W-CN bond in [W(CN)(8)](4-) in an aqueous solution under ambient conditions, in conjunction with the uptake of molecular oxygen, affords the W-VI mixed-ligand complex anion [W(CN)(7)(eta(2)-O-2)](3-), conveniently isolable as its [PPh4+] salt. Although research into the chemistry of cyanomolybdates and cyanotungstates has been pursued with great interest and vigor over several decades, there is a paucity of structurally characterized cyano-peroxo complexes of Mo and W. The side-on coordination mode of the peroxo moiety in [W(CN)(7)(A(2)-O-2)](3-) has been ascertained with X-ray crystal structure determination [d(O-O)) 1.41 angstrom; peroxo bite angle: 41.0 degrees] and corroborated with vibrational spectroscopy [v(O-O) = 915 cm(-1)]. The complex ion exhibits trapezoidal tridecahedral geometry and represents the new class of nine-coordinate complexes with one bidentate and seven monodentate ligands. Cyclic voltammetry shows a reversible redox behavior of [W(CN)(7)(eta(2)-O-2)](3-) in CH3CN with its standard reduction potential equal to 1.130 V. Generally, interest in atmospheric oxygen derives from the versatility of this molecule as a ligand and oxidant and extends to the physicochemical features it imparts to transition metals such as copper and iron in biological oxygen carriers.