Diesel vehicles are a major source of fine, atmospheric particulate matter in urban environments. The influences diesel particles exert on solar radiation, on atmospheric chemistry, and on humans depend crucially on the size and chemical character of these particles. In this work, size-resolved diesel particle chemistry has been examined by collecting particles directly from a diesel car exhaust with a low-pressure impactor. The impactor samples have been weighed and analyzed chemically to construct continuous size distributions for selected compounds present in the particulate phase. Submicron diesel-particle mass size distributions displayed three log-normal modes that were centered at 0.09, 0.2, and 0.7−1 μm of particle aerodynamic diameter (EAD) and that had average geometric standard deviations of 1.34, 1.61, and 1.34, respectively. The lowest two modes had approximately the same particulate mass, whereas over 80% of the number of particles were estimated to be found in the mode around 0.1 μm. The third mode contained about 10% of the total particulate mass but less than 0.1% of the particles. The size distributions of elemental (EC) and organic carbon (OC) were quite different:  EC peaked at 0.1 μm, and OC peaked somewhere between 0.1 and 0.3 μm of EAD. The mass ratios of OC to EC were between 0.3 and 0.5 in the bulk of the samples but were considerably lower for most of the particles. The presence of a catalytic converter reduced particulate mass by 10−30%, with the removal being more efficient for OC than EC. The principal mechanism producing the mode around 0.1 μm was shown to be Brownian coagulation between small primary particles formed during the combustion. The two larger size modes in the submicron particle range were hypothesized to be formed by activation and subsequent uptake of condensable organic compounds by some of the mode 1 particles.