Fuels in modern diesel engine fuel systems are exposed to highly oxidizing conditions, thus it is important to understand their degradation mechanisms. A range of chemical and physical properties was monitored during the oxidation and polymerization of soybean methyl ester biodiesel (B100), a diesel fuel (B0), and their blends (B10, B30) at 90 degrees C with aeration. The initial rapid oxidation of polyunsaturated fatty acid methyl esters (FAMEs) provided a transient pool of peroxides that led to the formation of aldehydes, ketones, and acids as secondary products. Monounsaturated and saturated FAMEs were oxidized concurrently with polyunsaturated FAMEs in B10, B30, and B100, but only B100 showed significant oxidation reactions continuing after the polyunsaturated FAMEs were depleted. New esters were a major oxidation product, eventually comprising 40-60% of the incorporated oxygen. Carboxylic acids and alcohols react to form esters and water, with vaporization of water driving the equilibrium toward ester formation. Polymers with ester linkages are likely contributors to the higher molecular weight materials formed and resulting increase in viscosity under these conditions.