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The effect of Cu loading on the selective catalytic reduction of NO x by NH 3 was examined over a series of Cu ion-exchanged (20-80%) SSZ-13 zeolite catalysts. High NO reduction efficiencies (80-95%) were obtained over all catalyst samples between 250 and 500°C, and at the gas hourly space velocity of 200,000 h -1 . Both NO reduction and NH 3 oxidation activities under these conditions were found to increase slightly with increasing Cu loading at low temperatures. However, NO reduction activity was suppressed with increasing Cu loadings at high temperatures ([500°C) due to excess NH 3 oxidation. The optimum Cu ion exchange level appears to be *40-60% since higher than 80% NO reduction efficiency was obtained over 50% Cu ion-exchanged SSZ-13 up to 600°C. The NO oxidation activity of Cu-SSZ-13 was found to be low regardless of Cu loading, although it was somewhat improved with increasing Cu ion exchange level at high temperatures. During the ''fast'' SCR (i.e., NO/ NO 2 = 1), only a slight improvement in NO x reduction activity was obtained for Cu-SSZ-13. Regardless of Cu loading, near 100% selectivity to N 2 was observed; only a very small amount of N 2 O was produced even in the presence of NO 2 . The apparent activation energies for NO oxidation and NO SCR were estimated to be *58 and *41 kJ/mol, respectively.