Biodiesel is the fatty acid alkyl esters produced by the transesterification of vegetable, animal or microbial lipids. After ethanol, it accounts for the largest proportion of global biofuel production. Yet, due to the level of polyunsaturation, biodiesel is also oxidatively unstable. When biodiesel oxidises the viscosity increases, which leads to reduced fuel performance and in extreme cases can lead to engine failure. To aid in understanding the process of this degradation a specialist NMR tube rig was designed to assess the oxidation of biodiesel. The NMR tube rig allowed the in situ H-1 NMR measurement of the sample while air was bubbled through at fixed intervals. The methyl esters of linolenic acid (18:3), linoleic acid (18:2) and oleic acid (18:1) were oxidised at 110 degrees C over a 24 h period. The decomposition of biodiesel is complex, and there is more than one mechanism involved in the degradation. Using this rig the onset of oxidation for 18:3 and 18:2 was found to be almost instantaneous. The rate of oxidation was found to be slightly less for 18:2 than 18:3 while the maximum rate was observed for 18:3 from the beginning of the oxidation, this was only observed after 280 min for 18:2. The oxidation of 18:1 started at approximately 500 min and, slowly degraded during the remaining reaction time. The formation of a number of secondary oxidation products such as aldehydes, ketones, alcohols and formates were also quantified. (C) 2012 Elsevier Ltd. All rights reserved.