The effects of sunlight on aqueous redox reactions between manganese oxides (MnOx) and selected organic substances are reported. No sunlight-induced rate enhancement was observed for the MnOx oxidation of substituted phenols, anisole, o-dichlorobenzene, or p-chloroaniline. On the other hand, solar radiation did accelerate the reduction of manganese oxides by dissolved organic matter (DOM) from aquatic environments. The photoreduction of MnOx by DOM was little affected by molecular oxygen in air saturated water (250 microM), but was inhibited by 2,6-dichloroindophenol (0.5-6 microM), an excellent electron acceptor. MnOx reduction also was photosensitized by anthraquinone-2-sulfonate. These results indicate that the photoreduction probably involves electron transfer from excited states of sorbed DOM to the oxide surface. Wavelength studies indicated that ultraviolet-B radiation (280-320 nm) plays an important role in the photoreduction. (Copyright (c) 1991 by the American Chemical Society.)