Interest in therapeutic applications of carbon monoxide release to physiological targets has led us to explore a photochemical strategy for such CO delivery. Here, we describe the photoactivated carbon monoxide releasing moiety (photoCORM), W(CO)(5)(TPPTS)(3-) (1), an air-stable, water-soluble tungsten(O) carbonyl complex of the trianionic ligand tris(sulphonatophenyl)phosphine. Near-UV photolysis of 1 in an aqueous buffer solution leads to the high quantum yield release of a single CO, the formation of which has been verified by three analytical methodologies. Furthermore, in aerated media, additional CO is slowly released from the W(CO)(4)(H2O)(TPPTS)(3-) photoproduct owing to autoxidation of the tungsten center, Thus, 1 serves as a carbon monoxide releasing moiety both in the primary photochemical reaction and in the secondary reactions of the initially formed photoproduct. The three methodologies for quantifying CO release under these physiologically relevant conditions are also described.