The polycyclic aromatic hydrocarbon (PAH) composition of a diesel fuel and the exhaust emissions obtained from the fuel in a diesel vehicle driven on a chassis dynamometer have been determined by high resolution gas chromatography. The principal components of the fuel are fluorene, phenanthrene and their alkylated derivatives. One- to four-ring aromatic components of the exhaust emissions are predominately in the gas phase when sampled at 200 °C. The major aromatic components of the exhaust are toluene, phenylacetylene, styrene, indene, naphthalene, methylnaphthalenes and acenaphthalene. These are not present in the fuel in significant concentrations, implying that they are formed by the combustion process. This is in contrast to recent work in which unburnt fuel has been identified as the major source of two- to four-ring PAH in diesel exhaust.