An old and a new diesel engine and three diesel engine fuels were compared regarding the concentration of particles in diluted exhaust immediately after dilution and after 40 seconds of aging. The flow of dilution air was adjusted to give a particle concentration of about 2 mg/m3. Filters were weighed and analysed with a Scanning Electron Microscope. The mean volume of soot particles were in the range 0.01-0.05 ·m 3, the mean weight, 10-40 femtogram. The mean density of the particles was found to be 1.7. In the different experiments the densities of the particles varied considerably indicating complicated agglomeration and adsorption-desorption processes. The new engine emitted significantly lower concentrations of particles than the old engine when measured in the dilution tunnel. The new engine was not better than the old and worn engine when results obtained after aging of the particles in the chamber were compared. When both engines were run on fuels intended for densely populated areas the new engine was worse than the old and worn engine after aging of the particles. The conclusion is that motor and fuel development should be governed not only by traditional measurement of particular matter, but by measurements of aged soot particles as well.