As an attempt to improve the catalytic activity at higher reaction temperatures between 300-450℃, various mole ratios of WO₃ were added to V₂O₅/TiO₂ catalytic systems. And also, in order to suggest a new mixed oxide catalyst system for simultaneous removal of NOx and SOx from stationary sources, MoO₃-V₂O₅/TiO₂ catalysts were prepared by a conventional impregnation method together with a newly introduced method of surface fixation (non-aqueous solution method). In case of WO₃ addition, at higher reaction temperature range(300-450℃), WO₃ and WO₃-V₂O₅/TiO₂ catalysts showed significant high conversion in NO reduction with NH₃ while V₂O₅/TiO₂ catalyst showed a significant change in selectivity mainly due to the excess side reaction of NH₃ oxidation. This difference in selectivity due to NH₃ oxidation at high temperature is supposed to be associated with the difference in values of surface excess oxygen between WO₃ and V₂O₅ on titania. The surface acidities of tested catalysts were relatively well correlated with the % conversion of NO at 400℃. In case of MoO₃ addition, the catalytic activity for the simultaneous removal of NOx and SOx were quite enhanced by the addition of MoO₃ into V₂O₅/TiO₂ catalysts. The enhanced activities were responsible for the formation of Mo=O bond on the intermediate species produced by solid solutions on MoO₃-V₂O₅/TiO₂(aqueous). However, in the case of MoO₃-V₂O₅/TiO₂ (non-aqueous), the exact source of active site was not able to detect in IR spectra in spite of more enhanced activity was obtained in this study. After SO₂ contact, VOSO₄ is newly formed on the surface of catalyst, which supposed to be associated with the activity enhancement.