The physical and chemical character of diesel particulate (DP) has been determined. Electron microscopy (EM) and a gravimetric BET method (BET) deter-mined particle size and specific surface area (SSA). Two different diesel engine types (direct injection (DI) and indirect injection (IDI)) were run at similar operating conditions and DP was collected by filtration of the exhaust gas at temperature without dilution. An in-vacuo gravimetric thermal degassing (TD) apparatus has been constructed to extract adsorbed hydrocarbon volatiles from the DP during a slow heating program. The desorbed volatiles were trapped and analysed by gas chromatography (GC). The carbons were fuel- and oil-derived unburnt hydrocarbons (UHC), trapped within the slit-shaped micropores and ’ink-bottle’-shaped mesopores/surface of the aggregated diesel particles.