The relationship between Diesel fuel composition and that of the solvent organic fraction of Diesel particulates; was investigated for an old DI Petter engine and a current technology DI Perkins engine. Polynuclear aromatic compounds (PAC) were indentified using high resolution capillary column chromatography with a parallel triple detector system for polycyclic aromatic hydrocarbons (PAH), nitrogen containing PAH (PANH) and sulphur containing PAH (PASH). The Perkins engine gave emissions with much lower unburnt hydrocarbons than the Petter engine and this was shown to be due mainly to the much greater efficiency of burn out of low molecular weight fuel compounds including n-alkanes and PAH. It was conclusively shown for both engines that the bulk of the particulate solvent organic fraction (SOF), including the PAH fraction, was unburnt Fuel. However there was some evidence that high molecular weight five ring PAH may have an incylinder formation contribution and it is postulated that this could he due to pyrolysis of lower molecular weight unburnt fuel PAH.