The selective catalytic reduction of NOx with hydrocarbons (HC-SCR) on functionalized multiwalled carbon nanotube (fMWCNT)-supported metal catalysts was investigated using a transient technique, together with kinetic and adsorption measurements. Results from the transient studies provide an explanation for the characteristic volcano shape of the NOx conversion curves: below T-max, the temperature of maximum NOx conversion, the catalyst surface is covered by hydrocarbonaceous species, which results in the suppression of NOx reduction activity. Above T-max, O-2 adsorption becomes prevalent, favoring oxidation of both NO and the hydrocarbon. In an effort to understand the origin of the superior NOx reduction activity shown by 3:1 Pt-Rh/fMVVCNTs as compared to Pt/fMVVCNTs, Temperature Programmed Desorption (TPD) measurements were undertaken. Results indicate that hydrocarbon and/or hydrocarbon-derived species are more strongly adsorbed on the alloy than on Pt alone, while NO adsorption is weaker on the alloy than on Pt. This is suggested to give rise to a higher concentration of partially oxidized hydrocarbon intermediates on the surface of the Pt-Rh catalyst at the temperature of maximum deNO(x) activity, leading to higher NOx reduction activity.