The catalytic reduction of NO, in the typical operation temperatures and oxygen concentrations of diesel engines has been studied in the presence of V3W9Ti in a tubular flow reactor. The results have shown that the selective catalytic reduction is strongly affected by the oxygen concentration in low temperature range (150-275 degrees C). At higher temperatures, the reaction becomes independent of the O-2 concentration. The rate of the selective catalytic reduction of NO with ammonia may be considerably enhanced by converting part of the NO into NO2. DRIFT measurements have shown that NH3 and NO2 are adsorbed on the catalyst surface on the contrary of NO. The experiments have shown that the decrease in N-2 selectivity of the SCR reaction is mainly due to the SCO of ammonia and to the formation of nitrous oxide. (c) 2005 Elsevier B.V. All rights reserved.