Fuel-borne catalysts (FBC) have demonstrated efficacy as an important strategy for integrated diesel emission control. The research summarized herein provides new methodologies for the characterization of engine-out speciated emissions. These analytical tools provide new insights on the mode of action and chemical forms of metal emissions arising from use of a platinum and cerium based commercial FBC, both with and without a catalyzed diesel particulate filter.Characterization efforts addressed metal solubility (water, methanol and dichloromethane) and particle size and charge of the target species in the water and solvent extracts. Platinum and cerium species were quantified using state-of-the-art high resolution plasma mass spectrometry. Liquid-chromatography-triple quad mass spectrometry techniques were developed to quantify potential parent Pt-FBC in the PM extracts. Speciation was examined for emissions from cold and warm engine cycles collected from an engine dynamometer.Water soluble platinum (6-hour extractions, 0.45 μm filtered) species represented a small fraction (2.8 ± 1.5%) of total aerosol-associated platinum. A large fraction of the water soluble platinum (44 ± 8%) was present as colloidal-sized particles. A similar fraction (54 ± 6%) of soluble cerium was retained on the ultrafilters. The fraction of platinum associated with the PM that has the potential to form truly dissolved (<10 kDa; ∼2nm) species averaged 1.5 ± 0.3%. Using ion-chromatographic methods it was determined that from 33 to 54% (41.5 ± 6.8%) of the <10 kDa -sized platinum species were anionic in character, while in contrast virtually all (97.5 ± 1.2%) of the cerium species were cationic or neutral in nature. Methanol extractions (24-hour Soxhlet, 0.45 μm filtered) released significantly more platinum (28 ± 1.4% of total) and DCM extractions significantly less platinum (0.66 ± 0.12% of total) than in water extractions. The parent Pt-FBC was not detectable in either methanol or DCM extracts of the diesel PM. The general speciation characteristics of FBC-platinum revealed in this study are consistent with findings of other studies of emissions from gasoline/diesel catalysts and those of environmental reservoirs.