The photocatalytic reduction of mercury(II) to metallic mercury using irradiated titania powders has been studied in a continuous system. The influence of oxygen, pH, gas and liquid recirculation flows, mercury(II) and titania concentrations on the photocatalytic reduction of Hg(II) was evaluated. The photocatalytic reduction of mercury(II) varied with pH, oxygen and titania concentration. The reaction is first order with respect to the Hg(II) concentration. Metallic mercury deposited on titania affected the rate of mercury(II) reduction. An activation-deactivation model has been fitted to explain these phenomena.