The assessment of an integral catalytic reactor facilitating operando spectroscopic measurements on a monolith has been carried out using NH3-SCR on a vanadia-based catalyst as a probe reaction. The NH3-SCR mechanism is revisited by studying the adsorbed ammonia and ionically bound ammonium ions and their relations to Lewis and Bronsted acid sites during reaction. The simultaneous presence of molecular water and ammonia adsorbed on the surface is intrinsic to low temperature NH3-SCR, and their IR absorption bands overlap in the bending region around 1600 cm(-1). This has to be tackled in order to genuinely reproduce real reaction conditions and simultaneously extract relevant spectroscopic data of a working industrial monolithic catalyst. This operando study on a V2O5-WO3/TiO2-sepiolite monolith is performed without added water in the gas phase, but water formed upstream is adsorbed further down, and thus detected by FTIR spectroscopy. We observed that Multivariate Curve Resolution Alternating Least Squares (MCR-ALS) analysis of the 1620 cm(-1) envelope of peaks can successfully resolve the bands of adsorbed H2O and NH3 in the monolith during transients between NH3 and NO + O-2 flow. Their evolution along with that of surface ammonium ions indicates that generated water hydrolyzes molecularly dispersed vanadia species of the catalyst creating new Brunsted acid sites, and that adsorbed ammonia is either used for the SCR reaction, or converted into the ammonium ion, which then participates in the SCR reaction.