Soot is the largest component of contaminants found in the diesel engine lubricating oil. The soot enters lubricating oil mainly through thermophoretic deposition on the cylinder wall. Although the mechanism is still not fully understood, it is generally accepted that soot particles promote engine wear, reducing engine component service life, fuel efficiency and performance. This problem will be further exacerbated when more and more diesel engines use EGR to reduce NOx emissions and when lubricating oil consumption is drastically reduced to control particulate emissions.In this study, lubricating oil samples were taken from 7 different operating diesel engines. The size distribution and concentration of the diesel soot particles in the lubricants were investigated by methods of photosedimentation and quantitative spectrophotometry. The size distributions were compared to those of soot particles in the exhaust. It was found that the mass weighted median diameter of soot particles in the lubricating oil varies from 0.12 to 0.27 μm, and the geometrical standard deviation from 2.4 to 4.0, largely dependent on the type of the engines. The soot size distribution in the oil is in general similar to that in the exhaust. For a single cylinder diesel engine, soot particles were found entering the lubricating oil at a rate of approximately 4% of the total dry exhaust particulate mass emission rate, which in turn is about 0.1% of the fuel mass consumed.