The photocatalytic reduction of the pollutant Hg(II) salts using a TiO2-photocatalyst (Hombikat UV100) was studied in aqueous air-equilibrated suspensions of 2 g TiO2 per dm(3) of Hg(II) chloride solution (100 ppm) and irradiated by a medium pressure (UV-VIS) mercury lamp. The photoreduction of mercury salts was studied as a function of irradiation time, pH of the suspension, initial Hg(II) concentration and mass of the semiconductor in the suspension. The activity of Hombikat UV 100 was compared with the catalytic activity of the "traditional" TiO2-photocatalyst (Degussa P25). The reduction was found to be first-order in [Hg(II)]. An optimum mass of Hombikat UV 100 (2 g per dm(3) of suspension), where a maximum light absorption by TiO2, was ascertained. The effect of oxygen on the photoreduction was investigated by experiments using air-equilibrated or nitrogen-purged suspensions. The results showed that the process is more efficient in the absence of oxygen. Using 20% (v/v) methanol aqueous suspension, increased the photoreduction. Changing the anion of the mercuric salt by using nitrate instead of chloride, also increased the extent of photoreduction. Hombikat UV100 was more efficient than Degussa P25. possibly because of the higher surface area of the former catalyst.