The effect of vanadium content on the resistance to K+-deactivation of V2O5-WO3/TiO2 SCR catalyst in biomass-fired flue gas was investigated. Catalytic activity and ammonia oxidation were measured, and the properties of fresh and K+-deactivated catalysts were characterized by XRD, N-2 physisorption, H-2-TPR, NH3-TPD and NH3-DRIFT. The BET surface area decreases with increased vanadium content for both fresh and K+-poisoned samples, but it is not responsible for catalyst deactivation. Ammonia oxidation starts from 300 degrees C and becomes more important with increasing vanadium content and at higher temperature. K+ can inhibit ammonia oxidation, but inactivates the newly generated NO2 to be reduced. The increase of vanadium content reduces mainly the Lewis acid sites, while the amount of Bronsted acid sites increases. Monomeric and polymeric vanadium are the dominant species on the TiO2 support, and the amount of isolated vanadyl (V = O) species decreases with V2O5 content while the amount of V-OH species in polymeric vanadia increases. Isolated vanadyl species are advantageous to high-temperature catalytic activity while polymeric vanadia species increase ammonia and K+ adsorption. The (3 wt% V2O5)-WO3/TiO2 catalyst shows the best performance for both NO reduction and K+ resistance due to it containing both monomeric and polymeric vanadia species (or V = O and V-OH). Catalysts with 3 wt% V2O5 are preferable for flue gases with high alkali metal contents. Finally, the mechanism of reaction for different vanadium contents and corresponding K+-poisoning are also discussed.