The effect of tungsten and barium oxides on the activity and durability of V₂O₅/TiO₂ catalyst for NO reduction by NH₃ was examined. Tungsten enhanced the NO removal activity of V₂O₅ catalyst supported on sulfur-free TiO₂, while no effect of tungsten was observed for the V₂O₅ catalyst supported on sulfated TiO₂. The tungsten oxide promotes the formation of polymeric vanadate that is a strong active reaction site for NO reduction by NH₃. When both tungsten and sulfur simultaneously exist on the surface of V₂O₅/TiO₂, the sulfur species seems to play a more important role for NO removal activity than tungsten. The tungsten oxide on the V₂O₅/TiO₂ catalyst also enhances the activity for SO₂ oxidation by promoting the adsorption of SO₂, regardless of the presence of sulfur species on the catalyst surface. The NO removal activity of V₂O₅ catalyst supported on sulfur-free TiO₂ has been significantly reduced by barium oxide, mainly due to the formation of inactive VOBa compound through the strong interaction of vanadia with barium oxide. No change of NO removal activity over V₂O₅-BaO/sulfated TiO₂, however, was examined by the addition of barium oxide, since the structure of vanadium oxide was not altered on the surface of the sulfated TiO₂. The SO₂ oxidation reaction over V₂O₅-BaO/TiO₂ catalysts was significantly suppressed by the addition of barium oxide to the catalyst. The barium oxide seems to reduce the redox ability of vanadium oxide on the catalyst surface as well as the adsorption capacity of SO₂. Based on the temperature programmed reduction (TPR), Raman and XPS observations, the surface structure of vanadium and its interaction with tungsten and barium oxides has been illustrated when sulfur exists on the surface of TiO₂.