A kinetic investigation of the selective catalytic reduction (SCR) of nitric oxide (NO) by propylene (C3H6) was conducted over a 0.8 wt % Pt/Al2O3 catalyst in the temperature range of 200-500 degrees C. The results indicate the presence of two kinetically distinct regions at temperatures above and below the temperature of maximum NO reduction. The activation of the C3H6 is kinetically significant in the low-temperature region. Nitrogen selectivities are significantly different in the two regions and are primarily affected by the amount of C3H6 available for the NO-C3H6 reaction.