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Emissions from compression ignition (CI) engines are being placed under stricter regulations. The use of oxygenated fuels has been proposed as a means of complying with future emission levels. Previous research on oxygenates has suggested the presence of molecular oxygen within the fuel significantly reduces the formation of particulate matter (PM). The PM reduction is logarithmic and correlates with the weight percent of oxygen in the fuel and not with other properties such as chemical structure and volatility. However, many studies fail to correctly decouple the effects of oxygenates from other fuel properties that impact emission levels; such as cetane number, density, aromatic content, and volatility. Mechanisms responsible for changes in the combustion process and emissions were investigated with a matrix containing 14 fuels, addressing oxygenate proportionality, structure, volatility, equivalent structure without oxygen, and changes in the base diesel fuel. A scanning mobility particle sizer (SMPS) was used to evaluate changes in particle size distributions, in addition to gravimetric filter and oxides of nitrogen (NOx) measurements. Engine emissions tests were conducted using a Ricardo Hydra MK IV direct injectio