The photoreduction of mercury (Hg2+ to Hg0) in natural seawater was investigated by means of a radiotracer (203Hg2+) solution exposed to natural and simulated sunlight. Different light regimes (dark, natural daylight, and a solar simulator), and dissolved organic carbon (DOC) levels from commercially available humic acids concentrations, were tested in the laboratory to evaluate the possibility of occurrence of the reaction in the environment. The natural seawater prepared accordingly to each experimental condition was continuously purged of the Hg0 formed, which was then re-oxidised in an acid trap and determined. The use of a solar simulator permitted the test of light intensity and wavelength dependence of the process under investigation. The reaction is dependent on the concentration of DOC in the experimental solution, increasing light intensity and decreasing wavelength. Reduction rates were in the range of 0.04-2.2% h(-1) for the DOC concentrations and light regimes tested. The process might have geochemical implications for the cycling of mercury around the air-sea interface.