In this study, ferric oxalate is used to represent the photosensitive Fe(III) complexes as well as the diacid compounds which are at significant concentrations in cloud and rain droplets. Because of the common carboxylate functional group; ferric oxalate is also used as a model to represent humic substances found in natural water. UVA irradiation of aqueous acidic mercuric chloride (pH 1-4) in the presence of an excess of ferrioxalate results in partial reduction of the mercuric ion to elemental mercury. The pseudo-first-order rate constant "kobs" for the photoreduction reaction is pH-dependent as is the yield of residual Hg(II). When exposed to visible irradiation the rate is about 10 times slower and no reaction was observed in the dark. The inferred mechanism of photoreduction involves the reaction of Hg(II) with a secondary photoproduct, the strongly reducing radical anion CO2-*. In the presence of dissolved oxygen, competition for CO2-* between Hg(II) and O2 reduces the rate and efficiency of mercuric ion reduction. The O2-*/HO2 products do not reduce Hg(II). On the contrary, their disproportionation leads to the formation of H2O2 which causes a re-oxidation of Hg(0) at pH values of <or=4. Chloride ion decreases the rate of the reduction of Hg(II).