Low-temperature selective catalytic reduction (SCR) of NOx on urea-supported carbon xerogels is studied. The kinetics results show that the orders of reaction for NO and O-2 are 1 and 0.5 for NO reduction, while the orders of reaction for NO2 and O-2 are 1 and 0.03 for NO2 reduction, respectively. The apparent activation energies of NO and NO2 reduction are calculated to be -14.0 and 0.79 kJ/mol, respectively. Possible mechanisms are proposed, based on the hypothesis that there exist two kinds of active sites on the surface of carbon xerogel, including nonreductive and reductive ones. Spillover of the NO3 species generated on the nonreductive ones into the reductive ones which are subsequently oxidized by NO3 and the migration of Time NO3 species from nonreductive ones into the oxidized carbon sites are considered to be of great importance for the successive urea-SCR process and the formation of steady-state NOx removal period.