Cu-exchanged zeolites of the CHA structure are state-of-the-art catalysts for selective catalytic reduction of NOx with NH3 in diesel aftertreatment systems. However, these catalysts deactivate in the presence of SO2, which is a constituent of diesel exhaust gas. In this article, the deactivation behavior and mechanisms of a Cu-SAPO-34 catalyst were studied with reactor tests and DFT calculations. Exposure of the catalyst to two different SO2 concentrations and durations, but with the same total SO2 exposure, calculated as the product of partial pressure of SO2 and exposure time, lead to the same degree of deactivation. Exposure of the Cu-SAPO-34 catalyst to SO2 in the presence and absence of NO and NH3 at different temperatures between 200-600 degrees C showed different trends for the deactivation. Below 400 degrees C, the S/Cu ratio on the catalyst increased with temperature in absence of NO and NH3, while it decreased with increasing temperature in the presence of NO and NH3. This is explained by the ability of NO and NH3 to reduce Cu(II) to Cu(I). DFT calculations show that SO2 adsorbs more strongly on Cu(I) than on Cu (II). Above 400 degrees C, the S/Cu ratio decreased with temperature irrespective of the presence of NO and NH3. In all cases, the S/Cu ratio is lower than 1. This is not compatible with extensive deposition of ammonium sulfate when co-feeding SO2, H2O and NH3. A more likely explanation for the deactivation is that SO2 is mainly related to the Cu sites. This is further corroborated by DFT calculations showing that SO2 and SO3, which is possibly formed by oxidation of SO2 over Cu sites, interact similar with Cu in Cu-SAPO-34 and Cu-SSZ-13.