To reveal the nature of SO3 poisoning of Cu/SAPO-34 selective catalytic reduction (SCR) catalysts, CHA structure, copper species, and NOx conversion were probed. The Cu/SAPO-34 catalyst was treated with different ratios of SO3 to SOx (0, 6, 13, and 20%) at 250 degrees C. The breakage of Si-O(H)-Al bonds takes place and leads to aluminum sulfate formation upon SOx sulfation. More isolated Cu2+ transform to copper sulfate with an increment of the SO3/SOx ratio. Catalysts poisoned with SOx show lower NOx conversion at 150-300 degrees C, the reason for which is a reduced number of isolated Cu2+ sites, as their turnover frequencies (TOFs) are identical. The loss of isolated Cu2+ and Si-O(H)-Al bonds is responsible for the inferior activity above 300 degrees C. The presence of SO3 cannot be ignored because of the irreversible reaction with Cu/SAPO-34 when diesel oxidation catalysts are applied upstream of the SCR. (C) 2017 Elsevier Inc. All rights reserved.