Interest has been growing in many countries in the potential use of diesel particulate filters (DPF). This type of after treatment technology has been shown to make very significant reductions in both the mass of particulate emitted in diesel exhaust gas, and also in the number of fine particulates, which have been linked in recent years with concerns for human health.Work carried out during a development programme investigating the capability of fuel soluble metallic additives to assist DPF regeneration, indicated superior performance from a novel combination of metals in fuel soluble form. Earlier work showed that a fuel soluble combination of organo-metallic additives based on sodium and strontium gave very effective regeneration characteristics, and was capable of burning out carbon at temperatures from about 160°C. However, it was known that the presence of sodium could lead to high temperature degradation of ceramic filter substrate materials, and this represented a limiting factor in the exploitation of this technology. Combinations of iron and strontium were shown to perform better than sodium and strontium, with the added benefit that there appeared to be no adverse effect on ceramic substrates at high temperatures.Exhaust particle size measurements were made both engine out, and post DPF, in treated and untreated fuels showing that there were no harmful side effects from the use of the iron-strontium combination in the fuel. Performance data for both additive systems, determined on a European light duty diesel engine, are given in the paper. No harm testing was carried out using a different type of engine. Work was therefore carried out on two types of engine, representing different combustion chamber designs.