The effect of NO on the activity of a Cu/V/K/Cl based catalytic filter in the combustion of soot was investigated. Temperature Programmed Oxidation (TPO) of soot, collected on the filter at the exhaust of a gas-oil burner, was performed in a flow microreactor both in the absence and in the presence of NO. Similar tests were also carried out with uncatalytic filter. For the catalytic filter the presence of NO resulted in a substantial increase of the rate of carbon combustion, while for the uncatalytic filter the effect of NO was negligible. The influence of NO and O-2 concentration in the gas fed to the reactor on the overall activity of the catalyst was also investigated. The results suggest that in the presence of NO the enhancement of soot catalytic oxidation is mainly due to carbon oxidation by NO2 in turn formed by NO oxidation on the same catalyst.