Shell companies* and Mercedes-Benz have investigated the effect of diesel fuel properties on exhaust emissions in an advanced European IDI passenger car and a modern commercial vehicle DI engine. The experimental programme identified the key fuel properties that influence diesel engine emissions with specific emphasis on NOX and particulates emissions. The fuel properties investigated were density, sulphur, distillation range, cetane number, mono aromatics and polyaromatics content. With regards to the polyaromatics content, the fuels were not designed to differentiate between the effect of di- and tri+ -aromatics.It is concluded that the fuel properties which account for the observed fuel effects on particulates emissions are sulphur, density and polyaromatics content. Monoaromatics content, cetane number and the distillation, as described by T10E, T50E and T90E, were found to have no effect on particulates emissions. Therefore total aromatics content is not helpful in describing fuel effects on emissions. The conclusions are valid for both the passenger car and the commercial vehicle engine, in the corresponding European test cycles, ECE+EUDC and ECE R49 (13-mode test) respectively.With regards to NOX emissions in the passenger car, no significant fuel effects were observed. In the commercial vehicle engine increasing cetane number reduced NOX emissions, whereas total aromatics content had no influence on NOX emissions.The findings provide a further contribution to the understanding of diesel fuel effects on emissions and towards defining possible diesel fuels which, in combination with current and especially advanced engine technology would give reduced emissions. In addition, it shows that the oil and motor industries can assist each other to meet future legislation, by considering both engine technology and fuel quality in conjunction, thereby ensuring that an overall balance is achieved in terms of production cost, energy/fuel consumption, fuel availability and CO2 emissions. This conclusion relates to engines as well as to fuels.