One of the more objectionable aspects of the use of the diesel engine is its emission of particulate matter. Methods for collecting particulate matter samples in the undiluted exhaust gases with an Andersen Impactor for gravimetric and electron microscopy analysis are developed. A direct injection Vee-eight naturally aspirated diesel engine was used in the study. This paper presents the results of an in-depth study of the physical characteristics of diesel particles. The size distribution of the particulate matter was obtained using an Andersen Inertial Impactor for the engine conditions applicable to the SAE 13-mode cycle. The particulate matter was analyzed using both scanning and transmission electron microscopes and was found to be comprised of individual spherical particles ranging from 100 Å† to 800 Å with a mean size of approximately 260 Å.The particulate matter was analyzed for carbon, hydrogen and nitrogen. The solid particulate matter collected within the Andersen Sampler had a carbon/hydrogen ratio that is temperature dependent ranging from .75 at an exhaust temperature of 100 C to approximately 5. at exhaust temperatures greater than 300 C. A correlation between the number of individual spherical particles in an agglomerate and the agglomerate size reveals a particle number range from a single, 100 Å particle to an agglomerate 30 μm in diameter contining an average of 4000 of the individual spherical particles.