Deactivation of titania-supported vanadia commercial SCR catalysts exposed to flue gases from both coal and coal-biomass co-firing boilers were investigated. BET surface area and average pore diameter measurements on both fresh and exposed commercial catalyst samples indicated pore plugging occurred to exposed catalyst samples. ESEM analyses showed fouling on catalyst surface, and poison deposition on both catalyst surface and inner pores. Activity assessments of commercial monolith catalysts with various exposures (time and fuel type) indicated that catalyst deactivation involves fouling, pore plugging, and poisoning. Different mechanisms may dominate depending on exposure time, catalyst properties, and combustion environment. Better controlled lab-scale investigations involved poisoning and sulfation of SCR vanadia/titania catalysts synthesized with an incipient impregnation method. In situ FTIR spectroscopy indicate that K, Na, and Ca (among others materials) reduce, and sulfation and tungsten increase ammonia adsorption intensity on Brønsted acid sites. Activity measurements by MS showed K, Na, and Ca poison SCR catalysts, and sulfation and tungsten enhance SCR NOx reduction activity. Both the decrease and increase of catalyst activity arise from the decrease and increase of the pre-exponential factor (A) correspondingly. Moreover, the decrease of NO reduction activity from each poison are consistent with the IR peak area decrease of ammonia adsorbed on Brønsted acid site caused by the corresponding poison but not Lewis acid sites. Therefore, Brønsted acid sites participate more actively in SCR reaction than Lewis acid sites. However, Brønsted acid sites itself do not possess NOx reduction activity as indicated by zero NO conversion on 9% W/Ti which shows large amounts of Brønsted acid sites population. Therefore, dual acid (Brønsted) -redox (vanadia) sites are suggested to provide the active center during catalytic reduction cycle with weakly adsorbed or gas phase NOx reacts with surface adsorbed ammonia. In addition, in situ FTIR spectroscopy combined with XPS analyses indicate that sulfate does not form on vanadia sites but on titania sites.