Diesel engine exhaust pipe and incylinder deposits were analysed for the global fuel, lube oil, carbon and ash fractions for a range of diesel engines. A large SOF fraction, typically 30%, was found and this was dominated by lubricating oil. These deposits are shown to contain significant levels of PAH and hence provide a source of diesel PAH emissions and possible sites for incylinder pyrosynthesis of high molecular weight PAH. A Perkins 4-236 NA DI was used to investigate the role of exhaust pipe deposits on PAH emissions. It was shown that PAH compounds could be volatilised from the exhaust pipe. The difference in the exhaust inlet and outlet particulate composition for diesel and kerosene fuels was used to quantify the n-alkane and PAH emissions originating from the exhaust pipe deposits. Comparison with pure PAH free fuels showed that the exhaust outlet PAH composition was similar to that expected from the exhaust pipe deposits. This indicated that incylinder sources of PAH was small for these pure fuels.