An Andersen Impactor was used to collect particulate samples in both the undiluted and diluted exhaust from a Caterpillar 3150 diesel engine operated on the EPA 13-mode cycle. A total of 24 samples were examined using the transmission electron microscope and approximately 300 photomicrographs were taken. The microscope analysis and photomicrographs revealed details concerning the physical characteristics of the particulate and permitted a direct visual comparison of the samples collected.The photomicrographs were used to obtain diameter measurements of the basic individual spherical particles that comprise the much larger aggregates/agglomerates. Nearly 11,000 basic particles were measured and the observed range of diameters was 70-1200 Å. The mean particle diameters in the undiluted and diluted exhaust samples were 479 Å and 436 Å respectively. respectively. A respectively. 436 A respectively. These size measurements were grouped, by operating mode, to provide size distributions for the basic spherical particles.At the lower temperatures of the diluted exhaust, evidence of hydrocarbon condensation is illustrated in many of the photomicrographs. In some instances, the condensed coating is so heavy as to completely obscure all detail of the aggregate/agglomerate on which it is adsorbed and the differences between undiluted and diluted exhaust samples are quickly discerned. Several of the samples showed what appeared to be “droplets” or “pools of liquid”.