Sulfur oxides are a common source for the deactivation of Cu-exchanged CHA zeolite based catalysts used for NOx reduction in diesel exhausts by selective catalytic reduction with NH3 (NH3-SCR). Since water and possible formation of SO3 affect the deactivation of Cu-CHA catalysts, the deactivation in the presence of SO2 or a mixture of SO2 and SO3 was studied by measuring the SCR activity in wet and dry gas at 200 and 550 degrees C. The estimated S-content in the catalysts before and after 4 h regeneration at 550 degrees C in NO, NH3, O-2 and H2O was related to the deactivation. The deactivation can be divided into two parts: a reversible deactivation that is restored by the regeneration treatment, and an irreversible part. The irreversible deactivation does not affect the activation energy for NH3-SCR and display a 1:1 correlation with the S-content, consistent with deactivation by Cu-sulfate formation. The reversible deactivation results in a lower activation energy and a deactivation that is larger than expected from the S-content. The presence of SO3 at 200 degrees C leads to higher reversible and irreversible deactivation, but has no significant impact at 550 degrees C. Furthermore, the irreversible deactivation is always higher when exposed at 200 degrees C than at 550 degrees C, and in wet conditions, compared to a dry feed. The deactivation is predominantly reversible, making regeneration at 550 degrees C a realistic approach to handle S-poisoning in exhaust systems.