emissions will need to be controlled from gas engines, which in the future could be using a syngas/producer gas as a fuel (generated from the gasification of biomass). In anticipation of this need, the activity of a copper-exchanged ultrastabilized zeolite Y was investigated, for the selective catalytic reduction of . The catalyst performance and properties were compared with reference samples of Cu-exchanged un-stabilized Y and Cu exchanged ZSM-5 zeolites. Activity was related to the number and nature of active centers, which were determined by in situ FTIR and TPD experiments using CO, NO, and as probe molecules. In the test reaction the ultrastabilized Cu-USY catalyst showed high activity at low temperatures and offered high hydrothermal resistance, whilst the Cu-ZSM-5 catalyst demonstrated the highest overall activity. The observed low temperature activity of the ultrastabilized Cu-USY catalyst was linked to the high abundance of sites, which were detected by in situ FTIR analyses using CO adsorption.