The activities of Pt/SiO2 and Pt/Al2O3 samples with comparable Pt dispersions for the reduction of NOX under lean-burn conditions using C3H8 and C3H6 as the reductant is reported. C3H6 is more effective than C3H8 at reducing NOX with both catalysts. With C3H6, Pt/SiO2 exhibits greater deNO(X) activity than Pt/Al2O3, while with C3H8 the opposite was observed. Kinetics measurements show that at steady state during the C3H6-NO-O-2 reaction the surface is dominated by carbonaceous species, while during the C3H8-NO-O-2 reaction adsorbed oxygen is the main surface species. As a consequence, the surface of the catalyst is quite different for the two reactions (reduced versus oxidised) and this provides a rational explanation for apparent discrepancies in the reaction mechanisms which have been proposed.