Vanadia-based SCR (VSCR) catalysts can effectively remove NOx (NO and NO2) from diesel exhaust using urea hydrolysis produced NH3 as a reductant. VSCR catalysts are often used because of their excellent sulfur tolerance. However, ammonium (bi)sulfate-which could form from the combination of SO3 or H2SO4 with NH3 when a diesel engine is operated at low load with high sulfur diesel fuel-can physically poison VSCR catalysts and lead to decreased NOx conversion. In this study, the effects of 50 ppmv SO2 on the catalytic performance of a state-of-the-art VSCR catalyst were investigated using a lab reactor under simulated diesel engine operating conditions. Experiments were performed to investigate two distinct real-world diesel aftertreatment temperature regimes: (1) typical diesel exhaust temperatures of 473-573 K, and (2) temperatures as low as 317 K to simulate engine start-up and shut-down. It was found that the presence of 50 ppmv SO2 under SCR conditions at 473-5 73 K slightly increases NOx conversion. On the other hand, temporarily cooling the catalyst temperature to 317K in the presence of 50 ppmv SO2 under SCR conditions leads to significant catalyst deactivation. Results from kinetic analysis, temperature programmed desorption (TPD), diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) and BET surface area measurements indicate the physical poisoning of VSCR catalysts due to the formation of ammonium sulfate species during the cooling process which significantly decreases the number of catalytic sites for SCR reaction. This work indicates that during low temperature events, simulating engine start-up and shut-down, SO2 and NH3 can combine and be oxidized to ammonium sulfate species which initiate progressive VSCR deactivation via physical poisoning. (C) 2014 Elsevier B.V. All rights reserved.