The photocatalytic reduction of selenate Se(VI) ions was studied using unmodified TiO2 and Ag-loaded TiO2 (Ag- TiO2) photocatalysts. In the presence of formic acid, both the TiO2 and Ag-TiO2 photocatalysts were effective in reducing Se(VI). The reaction proceeded through the reduction of Se(VI) ions to elemental selenium Se and then to hydrogen selenide gas (H2Se). When unmodified TiO2 photocatalyst was used, the Se formed from the reduction of Se(VI) was further reduced to Se2- in the form of H2Se upon the exhaustion of Se(VI) in solution. In the presence of the Ag-TiO2 photocatalysts, hydrogen selenide gas was generated simultaneously with the reduction of Se(VI). It, vas found that the maximum Se(VI) reduction rate occurred at pH 3.5 and at a 0.5 at.% Ag loading while the maximum hydrogen selenide gas generation occurred at pH 3.5 and at 2.0 at.% Ag loading. The simultaneous reduction of Se(VI) to hydrogen selenide gas can be attributed to efficient charge separation due to the mediation of photogenerated electrons by the Ag particles. A mechanism is proposed in terms of the TiO2-Ag-Se electronic interaction during UV irradiation. (C) 2003 Elsevier Science B.V. All rights reserved.