The fate of nanoparticulate cerium oxide from the diesel fuel catalyst Envirox was studied from its presence in the additive to its transformations during combustion through its exposure to plants and soils using a broad range of analytical techniques. Envirox is a fuel-borne catalyst comprised of nanoparticles of cerium oxide suspended in kerosene. The particles suspended in Envirox were confirmed by synchrotron X-ray diffraction, dynamic light scattering, and electron microscopy to be 5-7 nm crystals of CeO2 present as 15 nm aggregates. Significant changes to the particles were induced by the combustion process, resulting in 50-300 nm euhedral crystals of CeO2 in the exhaust as discovered using high resolution transmission electron microscopy. Single particle electron diffraction of the emitted cerium oxide particles showed evidence of ordered oxygen vacancies, indicative of a superstructure. Variations in the engine operating load resulted in no significant differences in the emitted cerium oxide particles. The mobility through soils and impacts on the plant Brassica napus (dwarf essex rape) of the emitted cerium oxide were compared to small and large CeO2 nanoparticles as well as diesel particulate matter emissions with very low cerium. The small CeO2 nanoparticles exhibited high mobility through soils and significant uptake and translocation in the plants. The large CeO2 nanoparticles showed extremely low mobility in soils and no significant increase in cerium anywhere in the plants. Cerium emissions from a diesel engine utilizing Envirox was found to have moderate mobility through the soils as well as an increased association with the roots of the plants, though translocation of the cerium into the aboveground biomass was not statistically significant. Despite uptake and translocation of some materials by B. napus, exposure to these cerium sources at 100 ppm Ce in the topsoil showed no significant impacts on the growth or overall health of the plants when compared to unexposed control samples. This dissertation shows that CeO2 nanoparticles employed as catalysts suspended in diesel fuel are altered during their use resulting in changes to their mobility and interaction upon entering the environment. This dissertation lays the groundwork for a new approach to nanotoxicology. / Ph. D. / Understanding the environmental impacts – and subsequently the impacts on mankind – of the use of nanomaterials is an enormously complex problem. The bottomup approach, whereby one can predict impacts from fundamental principals, is not practical because nanotechnology implementation into products is occurring far too rapidly and it is impossible for environmental toxicologists to keep pace. The properties of a nanomaterial are controlled by small changes to its physical/chemical properties that can be tuned to suit many different practical applications. During their use and subsequent release into the natural environment, nanomaterials are exposed to incredibly complex spaces that are capable of modifying the original nanomaterial still further. Thus, the originally produced nanomaterial will continue to evolve, and therefore change in its interaction with biotic and abiotic systems. In this dissertation, we examine the use of nanoparticulate cerium oxide in fuelborne catalysts as a case study. As a fuel-borne catalyst, nanoparticulate cerium oxide is employed to reduce carbon dioxide (a greenhouse gas and contributor to global climate change) and particulate matter (a known carcinogen) emissions from combustion in a diesel engine. Fuel-borne catalysts achieve this through the suspension of cerium oxide nanoparticles in the fuel, which go through the combustion process and exit the tailpipe with the rest of the diesel exhaust. Concerns over the emission of this emerging contaminant have resulted in its limited market penetration. Here we show that the nanoparticulate cerium oxide in fuel-borne catalysts is substantially altered by the combustion process and is emitted as significantly larger particles in the exhaust. We suspected that the emitted cerium oxide would have different behavior in the environment from previously studied, laboratory synthesized cerium oxide nanoparticles. The behavior of emitted cerium oxide was compared with that of laboratory synthesized cerium oxide nanoparticles by exposure to the plant Brassica napus. The exposure experiments showed that cerium oxide emitted from the combustion of a fuel-borne catalyst did indeed behave differently in the environment, though none of the exposures proved toxic to the plants at the realistic concentrations utilized in the study.