The kinetics of NH3 -adsorption -desorption over model V2O5-(WO3)/TiO2 catalysts for the Selective Catalytic Reduction of NOx has been studied by transient response techniques, and described according to rate expressions which account for heterogeneity of the catalyst surface. A dynamic mathematical model of the industrial SCR monolith reactor has been derived, using the results of the transient study of NH3 adsorption as well as other independently estimated parameters. It has been successfully validated against laboratory data concerning unsteady operation of commercial SCR honeycomb catalysts, and applied to the simulation of typical transients of industrial SCR reactors. It is shown that the variations of the NOx emissions are always much faster than the dynamics of NH3.