Abstract: A series of Nb-modified V₂O₅-WO₃/TiO₂ catalysts were prepared by the impregnation method and the effect of Nb loading on their SO₂ oxidation activity during the selective catalytic reduction of NO_x was investigated. The results indicate that the Nb₂O₅-V₂O₅-WO₃/TiO₂ catalyst with a Nb₂O₅ loading of 2% exhibits the lowest SO₂ conversion of 0.6% for oxidation at 350 °C, whereas the conversion of NO_x is still above 95%. The catalysts were characterized by TGA, BET, XRD, H₂-TPR, CO₂-TPD, XPS and in-situ DRIFTS. The results illustrate that the influence of Nb modification on the crystal structure of V₂O₅-WO₃/TiO₂ catalyst is rather insignificant; however, the surface area of the Nb₂O₅-V₂O₅-WO₃/TiO₂ catalyst decreases slightly after the modification with Nb, conducing to a decrease of SO₂ adsorption on the catalyst. Meanwhile, the content of oxygen adsorbed on the catalyst surface decreases considerably upon the Nb modification, suggesting a weakened redox performance, which is beneficial to reducing the oxidation of SO₂. The in-situ DRIFTS results illustrate that the content of the intermediate VOSO₄ product on the catalyst surface decreases over the Nb-modified Nb₂O₅-V₂O₅-WO₃/TiO₂ catalyst, leading to a decrease of SO₃ production.