The reduction of particulate emissions from diesel engines is one of the most challenging problems associated with exhaust pollution control, second only to the control of NOx from any “lean burn” application.Particulate emissions can be controlled by adjustments to the combustion parameters of a diesel engine but these measures normally result in increased emissions of oxides of nitrogen. Diesel particulate filters (DPFs) hold out the prospect of substantially reducing regulated particulate emissions and the task of actually removing the particles from the exhaust gas has been solved by the development of effective filtration materials. The question of the reliable regeneration of these filters in situ, however, remains a difficult hurdle. Many of the solutions proposed to date suffer from high engineering complexity and/or high energy demand. In addition some have special disadvantages under certain operating conditions. A new approach to DPF regeneration is the utilisation of the capability of nitrogen dioxide (NO2) to oxidise carbon. Normally though, the concentration of NO2 in the raw exhaust gas is too low to oxidise the soot trapped in a DPF. In a continuously regenerating system reported in 1 the concentration of NO2 in the exhaust gas is increased by the use of a Platinum-based oxidation catalyst installed upstream of a cordierite wallflow particulate filter. As the exhaust gas passes through the oxidation catalyst some NO is converted to NO2 which flows over the soot trapped in the filter continuously oxidising it whenever the engine is in normal use.This paper describes the effect of such a continuously regenerating system on regulated and non-regulated exhaust emissions. Because of the rising concern over the contribution of small particles to health risks, ascribed in many epidemiological studies to diesel derived particles [2,3,4], special attention has been paid in this study to the effect of the system on particle size distribution. The work described was carried out on a turbocharged heavy duty HSDI diesel engine with a EURO 2 calibration. The system tested showed good performance in reducing regulated gaseous emissions and particulate mass emissions. Non-regulated gaseous emissions like aldehydes were also substantially reduced. For most hydrocarbon species, concentrations downstream the DPF system were too low to be measured. Particle number emissions were recorded down to a primary diameter of 15 nm and these emissions were also substantially reduced throughout the measured size range. Continuous DPF regeneration was found to occur at all operating points tested within the scope of this work, which varied from idle to rated power, resulting in a very stable exhaust backpressure level.