Particulate Matter (PM) influences both global climate and public health. For this reason, regulators have set stringent standards for ambient PM concentration. Often, these standards are difficult to meet in large urban centres. As a result, considerable effort is being put into the identification of the sources of atmospheric particles to tackle the problem at its origin. Most anthropogenic PM is emitted from combustion sources. Among other compounds, these combustion processes produce a range of particle bound polycyclic aromatic hydrocarbons (PAHs); the relative concentrations of these compounds may be characteristic of the source, fuel and combustion conditions. This is particularly the case for diesel engines, one of the major sources of PM in urban locations. In this project, 40 samples were collected in Sydney and an additional 20 samples were received from Hanoi, Vietnam. After soxhlet-extraction in an organic solvent, the extracts were concentrated and analysed by GC/MS using internal deuterated standards to quantify 16 PAHs. The different diesel-specific diagnostic ratios used in the literature were calculated. In order to assess the validity of these ratios, the PAHs concentrations of a previous Environment Australia study on emissions from diesel vehicles in Sydney were used to calculated the same ratios. It was found that some of the ratios previously used to characterise diesel emissions were more applicable than others to Sydney’s specific fuel and fleet conditions. In Sydney, it was possible to relate diesel emissions to specific atmospheric conditions. For Vietnam, however, the conclusions were not as clear, as there was no locally available information on PAHs from diesel emissions.