Catalytic activities of copper-exchanged ZSM-zeolites for the reduction of nitric oxide to nitrogen using ethylene as reducing agent in the presence or absence of oxygen are compared with those of Rh-ZSM-5 zeolites and binary Cu/Rh-ZSM-5 catalysts at 623-823 K. Copper ZSM-5 systems with high copper loadings (> 2.1) are shown to be much more active in the presence of oxygen, in marked contrast to Rh ZSM-5 catalysts which were more active in its absence. Binary ZSM-5 catalysts containing both copper and rhodium maintained intermediate activity in both conditions. A sample of Cu ZSM-5 containing only 1.8% copper also evidenced some activity in both conditions, which could be understood on the basis of catalytic action by copper ions within the zeolitic framework, allied to inherent Bronsted acidity of the latter. Comparative catalytic data are presented for the oxygen partial pressure dependence of the conversion to nitrogen over all catalysts at 673 K. These delineate the interplay between selective catalytic reduction (SCR) of NO by C2H4 and ethylene oxidation by dioxygen. Techniques utilised to characterise the catalysts include : powder XRD, TPR in H-2, and TPD of O-2.