Diesel particulate filters (DPF) are used to remove particulate matter (PM) from diesel engine exhaust. DPFs are regenerated periodically to oxidize the soot, leaving behind ash, which is the non-combustible portion of the PM. Ash build up in the DPF causes an increase in back pressure. This in turn, decreases fuel economy, increases cleaning frequency and contributes to degradation of DPF performance. Primary sources of diesel ash emissions are metallic additives present in the lube oil, fuel borne additives and engine wear metals.The high temperature oxidation method (HTOM) was developed at the University of Minnesota to measure ash emissions in real time. HTOM utilizes a tube oven that combusts the PM present in the exhaust leaving behind the non-combustible portion, which is measured in real time with instruments such as the engine exhaust particle sizer (EEPS).A series of experiments were performed to examine survival of ash components passing through the oven and the morphology of the particles leaving the oven. Morphology and limited composition information were obtained by collecting samples for Transmission Electron Microscopy (TEM) and Energy Dispersive Spectroscopy (EDS) analysis. The aerosol sources used for these experiments were sprays of lubricating oils with specially formulated additive packages and aqueous sprays of likely ash constituents. Atomized lube oils were only used for qualitative experiments because it was difficult to determine the loss of volatiles and thus the concentration additives upstream of the oven. TEM images of the samples indicated that calcium additized oils, containing 3700-3900 ppm Ca, produced particles in the size range of 5-50 nm; while a magnesium additized oil containing 500 ppm Mg, displayed a size range of 5-30 nm. Zinc additized oil, containing 1000 ppm Zn produced the lowest concentrations downstream of the oven and the smallest particles, in the size range of 5-20 nm. The aqueous salt spray experiments were conducted to determine the survival of likely ash constituents in the HTOM. Salts of calcium, magnesium and zinc were atomized in water solutions and particle concentrations measured upstream and downstream of the oven. Survival fraction for the salts were found to be 54.7% for CaSO4, 34.6% for MgSO4¬, 26% for Zn3(PO4)2 and 7% for ZnSO4. Experiments were then conducted using actual diesel engine exhaust particles as an aerosol source. The objectives of these experiments were to determine the morphology and composition of the particles leaving the oven and estimate the engine ash emissions. Engine ash samples were collected at 250 N-m torque load and 1400 RPM. Total particle concentrations were measured with an SMPS. The TEM samples showed particles in the size range 5-100 nm and EDS analysis indicated that they consisted of mainly calcium and oxygen. The SMPS measurements were corrected for losses assuming CaO was the main ash constituent and indicated an exhaust ash content of 44.3µg/m3 which corresponds to about 0.3 % of the exhaust PM.