An investigation into the impact of sulfur oxide on the activity of Cu-SAPO-34 towards selective catalytic reduction of NOx by NH3 has been conducted to clarify the possible mechanism of deactivation induced by sulfur. Several reactions including NH3 storage/TPD, NO/NH3 oxidations, standard and fast SCR, as well as SCR with an NO2/NOx ratio of 75% were performed at temperature range of 150-500 degrees C over the fresh Cu-SAPO-34 after it had been sulfated at 300 degrees C with 30 ppm SO2 in the presence of 8% O-2 and 5% H2O for 90 min. The catalyst is characterized by using XRD, BET, ICP-AES, H-2-TPR and micro-calorimetry. The BET surface area as well as the pore volume decreased after sulfur poisoning, hence some pores in the zeolite were blocked by sulfur. The standard SCR reaction was significantly influenced by the sulfur poisoning. The H-2-TPR data showed that there is less available copper that could undergo the redox cycle for the sulfated sample compared to the fresh sample and this could be the main reason for the deactivation seen. The conversion for NO during standard SCR showed a more pronounced decrease in activity compared to that of fast SCR and the smallest effect of the sulfur poisoning was observed for SCR with the 75% NO2/NOx. Hence, the SCR reactions in the presence of NO2 are less influenced by the sulfur on the surface and it is likely that the mechanism is different for SCR in the presence of NO2. Cu-SAPO-34 produced very small amounts of N2O and its production correlated with the amount of NO2 in the feed. From the calorimeter experiment, it was observed that the binding of SO2 is very strong on the catalyst sites, most likely the copper sites, and the heat of adsorption of SO2 was higher in the presence of O-2. (C) 2014 Elsevier B.V. All rights reserved.