The mechanisms of the selective catalytic reduction of NO with NH3 are under debate, especially in terms of the participation of Bronsted and Lewis acid sites on V2O3/TiO2 catalyst. Our results elucidated the dynamic nature of transformation between Bronsted and Lewis sites on V2O5/TiO2 occurring in SCR-relevant conditions through a series of operando DRIFTS experiments. The reversible change between the two acid sites occurred not only by hydration in the presence of water but also by changing the redox state of the catalyst. We proposed that under reducing conditions, the reduced vanadium sites coordinated with ammonia to serve as additional Lewis acid sites; as a result, the reducing environment enriched the surface with Lewis sites, while the oxidizing environment gave the catalyst surface more Bronsted sites. This in turn suggested that the equilibria between the two acid sites, that is, the B/L ratio, differ depending on the reactants contained in the reaction gas. In a simulated transient SCR reaction, the instantaneous change of the gaseous environment resulted in the redistribution of ammonia on both acid sites, which occurred much more quickly than the reaction with NO. Such observations emphasize the importance of clarifying a NH3-equilibrated surface under reaction conditions when measuring rates of ammonia consumption. (C) 2020 Elsevier Inc. All rights reserved.