To develop a methodology for characterizing particulate emissions from diesel engines, one 2-stroke cycle engine and one 4-stroke cycle engine were operated in both individual steady-state modes and according to a variation of the 13-mode diesel emissions measurement procedure. Both engines were operated on three fuels, each used with one of two available diesel fuel additives as well as by itself.The primary particulate sampling technique employed was a dilution tunnel, and secondary evaluation techniques included a diluter-sampler developed under contract to EPA by another organization, a light extinction smokemeter, and a filter-type sampling smokemeter. Gaseous emissions were also measured, providing a running check on engine condition.Particulate mass rates were calculated from gravimetric data; and analysis of particulate included determination of sulfur, carbon, hydrogen, nitrogen, phenols, nitrosamines, trace metals, and organic solubles. Analysis of the organic soluble fraction included NMR, IR, paraffin boiling point distribution, benz(a)pyrene, sulfur, carbon, hydrogen, nitrogen, and oxygen.The results of this study are the first of their type to be widely distributed, so it is imperative that the use of emissions values computed be treated with caution until other laboratories can confirm these findings. This caution is especially important for materials like nitrosamines and BaP, for which analysis is difficult and as yet not thoroughly documented in the literature as applied to emissions from diesels.