Samples of fine particle emissions from four heavy-duty diesel vehicles were analyzed for chemical and molecular composition. Significant variation was found for the composition of particle emissions from vehicles under load, with the fraction of carbonaceous particles emitted as elemental carbon (i.e., soot) varying between 19 and 94%. The emissions from vehicles idling shows little compositional variation with low soot formation and elemental carbon representing between 12 and 21% of total carbon emitted. This variation is important to consider when performing source apportionment calculations as the difference in fraction carbon emitted as elemental carbon is used to separate diesel emissions from gasoline-powered vehicle emissions and other sources. The molecular composition of the organic particulate matter was quantified for samples collected from each of the four vehicles under load plus a composite sample of all four vehicles while idling. The variation of the molecular composition of organic particulate matter was highest for polycyclic aromatic hydrocarbons and least for petroleum biomarkers, supporting the use of petroleum biomarkers as tracers for diesel vehicle emissions when the compositional information from sources is based on a limited sample set.