As part of a fundamental and applied work on the development of an unsteady mathematical model of the NH3-selective catalytic reduction (SCR) process for design and control of integrated after-treatment systems of heavy-duty engines, we present. herein a transient kinetic analysis of the standard SCR NO + NH3 system which provides new insight in the catalytic kinetics and mechanism prevailing at low temperatures. Based on kinetic runs performed over a commercial powdered V2O5-WO3-TiO2 catalyst in the 175-450 degrees C T-range feeding NH3 and NO (1000 ppm) in the presence of H2O (1-10%, v/v) and O-2 (2-6%, v/v), an original dual-site modified redox rate law is derived which effectively accounts for NH3 inhibition effects observed during transient reactive experiments at T < 250 degrees C. We also demonstrate that implementation of the novel modified redox kinetics into a fully predictive ID + ID model of SCR monolith reactors can significantly improve simulations of SCR transient runs at different scales, including engine test bench experiments over full-scale SCR honeycomb catalysts. (c) 2005 Elsevier B.V. All rights reserved.