Solar energy conversion to chemical potential energy is thermodynamically feasible by many routes. One possible route is the photochemical reaction of metal ions in water to produce hydrogen and an oxidizer. The photooxidation of several low-valent transition metal ions, including europium(II), vanadium(II), and copper(I) complexes, proceeds in aqueous acidic media according to: ? Quantum yields in 1.0 M hydrochloric or perchloric acid at 313 nm are: ΦEu(III) = 0.16, ΦV(III) = 0.15, and ΦCu(II) = 0.34. This reaction proceeds in visible light with a minimum of photochemical complications for Eu(II) and Cu(I) salts, and since the oxidation of copper(I) halo-complexes is endergic and hence potentially useful for energy storage, the mechanism of photooxidation has been studied. The product quantum yield is strongly affected by the acidity, irradiation wavelength and H-atom scavengers. Photoredox reactions of a number of metal ions and the requirements for using such in a solar energy scheme are discussed.