Sulfur poisoning and regeneration mechanisms for a 2% Ag/-Al2O3 catalyst for the H2-assisted selec- tive catalytic reduction of NOx by NH3 are investigated. The catalyst has medium sulfur tolerance at low temperatures, however a good capability of regeneration at 670◦C under lean conditions when H2 is present. These heating conditions can easily be established during soot filter regeneration. Furthermore, two types of active sites could be identified with different regeneration capabilities, namely finely dis- persed Ag and larger Ag nanoparticles. The most active sites are associated with the finely dispersed Ag. These sites are irreversibly poisoned and cannot be regenerated under driving conditions. On the other hand the larger Ag nanoparticles are reversibly poisoned by direct SOx adsorption. The interpretation of the data is supported by DFT calculations.