Particulate matter (PM) emissions from a common-rail automotive diesel engine were characterized as a function of the engine operating regime in terms of mass, particulate number, size distributions, and composition (soot and volatile fraction), as well as by means of the optical measurements currently used for engine calibration. Exhaust gas samples were taken both upstream and downstream of the diesel oxidation catalyst (DOC), so to evaluate its effect on PM modification. Moreover, the effects of the engine calibration were also investigated at two different exhaust gas recirculation (EGR) rates. Fundamental information on the PM: characteristics, which is essential for the knowledge-based design of a new generation of diesel particulate filters (DPFs), was gathered. The loading of a DPF entails the need of trap regeneration by particulate combustion, whose efficiency and frequency are somehow affected by the way soot is deposited along the channels. Small lab-scale 300 cpsi DPF samples were loaded downstream the DOC in an ad hoc designed reactor capable of hosting five samples with part of the entire flow produced by an automotive diesel engine at the 2000 x 5 mu BEP operating condition. Soot layer thickness was estimated by means of FESEM observations after sample sectioning at progressive locations obtained through a procedure defined not to affect the distribution of the soot inside the filter and to enable estimation of the actual soot thickness along the channel length.