Bicomponent Cu-Sm-ZSM-5 zeolites with different Cu and Sm contents have been investigated, and compared with monocomponent Cu-ZSM-5 and Sm-ZSM-5. Sm-ZSM-5 is almost inactive in NO decomposition, producing only N2O. Zeolites containing around 0.20 wt% Sm and between 2.2 and 2.7 wt% Cu are substantially more active than the pure Cu-ZSM-5, the increase in NO transformation per total Cu atoms reaching 100%. The catalysts were studied using XPS, O-2-TPD and FT-LR of adsorbed NO. In addition, the transient period between the first contact of the catalyst with NO and 3 h was monitored. The bicomponent Cu-Sm-ZSM-5 catalysts exhibit a very strong O-2-TPD band around 370 degrees C compared to the monocomponent catalysts. FT-IR shows a strong increase of NO adsorption in the (NO)(2) form, and a corresponding decrease of the bands of isolated adsorbed NO. The presence of Sm does not significantly prevent the loss of Cu dispersion during a heat treatment in helium, nor the Cu++/Cu+ ratio. Although it is impossible to conclude unequivocally, it seems that samarium has two roles. It modifies the exchange of copper with the zeolite and, consequently, the dispersion and aggregation of copper.