We measured the photosensitivity of an artificial tetranuclear oxo-Mn(IV) complex, [Mn(4)O(6)(bpea)(4)]Br(4), which has an adamantane-shaped {Mn(4)O(6)}(4+) core. Illumination caused changes in the absorption spectrum of the compound consistent with a one-electron reduction in the compound. Bromide appears to be the most probable electron donor in the reaction system. Chemical modification of the cluster appears to destabilize it, predisposing it to reductive degradation. UV light was more efficient than visible light in causing the changes. The data support the suggestion that the natural oxygen-evolving Mn complex is photosensitive and can oxidize components of the oxygen-evolving complex in its excited state causing photoinhibition, and that photostability is an important issue in designing Mn complexes for artificial photosynthesis. Furthermore, light-induced oxidation of bromide by [Mn(4)O(6)(bpea)(4)](4+) may suggest that oxidation of chloride is involved in natural water splitting or has been involved during the evolution of the water-splitting enzyme.