This program examined the PAH emissions from two diesel engines representing state-of-the-art technology. One heavy-duty engine, a DDC Series 60, used in highway trucks, and one medium-duty engine, Navistar DTA-466, used in pick-up and delivery type vehicles. The medium-duty engine was tested for engine out emissions and with two different sized exhaust oxidation catalysts. Both of the catalysts were composed of a ceramic monolith coated with a palladium type catalyst. Both engines were tested according to the US EPA transient emissions test on a series of diesel fuels blended with varying aromatics and PAH contents to determine the impact of fuel quality on emissions. All diesel fuels had a low sulphur content (<0.05%) to minimize the formation of sulphate.The particulate matter was collected on teflon-coated filters, backed up with polyurethane foam filters to capture the vapour phase PAH. The PAH's were extracted from both filters with dichloromethane and measured by high resolution gas chromatography (GC) coupled to a low resolution mass spectrometer. In addition, a number of nitro-PAH's were analyzed by GC coupled to a high resolution mass spectrometer. In this way individual PAH and nitro-PAH molecules could be identified. Of 30 PAH compounds used for reference purposes, 21 were confirmed as present in the diesel exhaust from some of the engine/fuel combinations. Those compounds ranged in complexity from 2-ring naphthalene type compounds up to 6-ring benzoperylene.Based on a comparison of results between four test fuels there appears to be no relationship between total fuel aromatics and the total measured PAH emissions. However, there was a correlation found between the level of polyaromatics in the fuel and the PAH emissions. For example: Fuel F, with the highest level of 3+ring aromatics, gave significantly more 3+PAH emissions than the other three fuels. The highest level of specific PAH emissions were found to be 2, 6-dimethylnaphthalene, phenanthrene and fluorene with Fuels A, B and E. Fuel F produced more phenanthrene, pyrene and 1-methylpyrene. While fuel PAH content was found to have an influence on PAH emissions, the results also suggest this is not the only source. It is suggested that a significant proportion of the PAH emissions comes from the lube oil or is formed in the combustion process. In these tests, results indicate about 50 #g/bhp-hr of the measured PAH emissions are formed in this way.An 8.5L palladium exhaust catalyst (a non-production unit) fitted to the Navistar DTA 466 engine reduced PAH emissions, by an average of 76%. Similarly, a 5.0L palladium catalyst (a unit intended for production) tested with Fuel B, on the same engine, reduced PAH emissions by 62%.The exhaust extracts were also analyzed for 7 specific nitro-PAH compounds. These were confirmed present, with 1-nitropyrene and 9-nitroanthracene being the most commonly found. The 8.5L catalyst fitted to the Navistar engine increased the level of nitro-PAH emissions with 3 of the fuels, Fuel A, B and F, by 31-118%. The 5.0L catalyst, tested with Fuel B, increased these emissions by 206%.Conventional, regulated emissions (HC, CO, NOx and PM) and aldehyde emissions were also measured during this test program.