Catalysts were prepared by exchange and co-exchange techniques following Iwamoto's procedure. Copper-exchange degree in the zeolites varied between 21.2% and 128% while the Sm content varied in inverse order, between 27.2% and 18.8%. The catalysts were characterized by NO-DRIFT, O-2-TPD, and XPS. Catalytic tests were performed in a microreactor in the temperature range 673-873 K. It was found that the presence of Cu in close proximity of the Al framework, i.e., in the exchangeable positions, is increased in the presence of Sm. The presence of Sm stabilizes the Cu(I)-O-Cu(II) "redox buffer," increasing the activity of this catalyst. This behavior is mainly observable for low Sm concentration.