Cetane number, sulphur content and aromatic structure of Automotive Diesel Oil (ADO) were changed to assess their influence on emissions of light duty direct injection Diesel engine. The detailed chemical analysis of particulate soluble fraction allows to quantify the P.A.Hs emission. In addition also the aldehydes and volatile organic compounds were measured in the gaseous phase.The sulphur content of the fuel and its aromatic structure strongly influence particulate emission. The insoluble fraction of the particulate rises with an increase of the high sulphur content ADOs with about the same back end volatility. Unburned P.A.Hs control P.A.Hs emission at the part loads typical of normalized schedules for emission testing of light duty vehicles in Europe. Finally the level of emissions of benzene and 1-3 butadiene is comparable to the total P.A.Hs emission.Given the more stringent limits for diesel engine emissions planned in the next years in Europe, the Automotive Diesel Oil (ADO) quality has come under scrutiny in that it affect emissions. Engine manufacturers demand now a good quality ADO available across Europe.The effect of the different fuel parameters on emissions is difficult to understand the response depending upon different engine technologies. Besides some important fuel variables are strictly connected what prevents from detecting separate effects on emissions. Extensive works aimed at improving the knowledge of those problems; Van Bechoven  synthesized the main results. On the contrary, more limited data are available on the effect of ADO quality changes on unregulated emissions like PAHs, Aldehydes and Volatile Organic compounds (V.O.C.) [4,5,6] that is mainly due to the difficulties related to performing the time consuming detailed chemical analysis of the samples for many test conditions. Moreover, the generalization of results is often limited because the effectiveness of both the sampling methods and sample treatment procedure is uncertain and properly designed fuel matrices are unavailable.To solve above problems, in 1989 the Institut Francais du Petrole (IFP) (France), the TNO (Netherlands), the Euron (Italy) and the Istituto Motori (Italy), under partly founding of the Commission of the European Communities, started a program whose aims are: a) to study the ADO composition parameters which influence particulate emissions; b) to define refining methods allowing to obtain clean diesel fuels .The present paper discusses some results obtained in Istituto Motori labs on a part of the total Joule fuel matrix: they are useful to assess the horigins of the main unregulated emissions.