The effect of steam injection and sedimentation treatment of waste cooking oil on the quality of TG, to be used as a raw material for the production of biodiesel, was investigated. The effect of steam treatment was evaluated in terms of a number of physical and chemical characteristics. Significant decreases in the moisture, FFA, and PV, as well as increased energy value, were observed. GC analysis of the treated materials demonstrated little change in either the overall composition of the oils or the iodine value. The decreases in moisture from 1.4 to 0.4% and in FFA from 6.27 to 4.28% were found to correlate strongly with an increase in yield of ester from 67.5 to 83. 5%. Paper no. J9938 in JAOCS 79, 175-178 (February 2002). KEY WORDS:Biodiesel, effect of steam treatment, ester yield, waste cooking oil.Biodiesel is the name applied to renewable fuels, manufactured by the alcoholysis or transesterification of vegetable oils or animal fats with methanol, that are used as a substitute for or as an additive to mineral diesel. The methyl ester produced using methanol and a basic catalyst is the major source of biodiesel available for compression ignition engines, as its properties closely resemble those of No. 2 mineral diesel. The European Union (EU) has set an objective of 5% of transport fuels to be produced from renewable resources by 2005, of which a substantial portion is expected to be biodiesel (1). Primarily owing to the competition with the food industry for available land as well as the price support mechanisms within the EU at present for vegetable oil, it is necessary to look at alternative sources of raw material in order for biodiesel production to become more economical (2). Waste cooking oil (WCO) offers some promise as an alternative low-cost biodiesel feedstock whose availability is not affected by EU land-use policies. Based on estimates from seven countries, a total of about 0.4 Mt is collected within the EU, mainly from the catering industry, while the amount that could be collected is estimated to be considerably higher, possibly from 0.7 to 1 Mt. Its price is variable, but in general is approximately half that of virgin oil (3). More recently the detection of dioxins in WCO used in animal feed rations, its primary market, has focused attention on this material and caused a major reappraisal of its use (4). At present, the use of waste oil in animal feeds has been voluntarily terminated in about half of the EU states.Biodiesel quality is directly related to the quality of the raw oil. During heating of vegetable oils for frying purposes, thermal, oxidative, or hydrolytic reactions can cause chemical changes (5). The compounds formed depend on the composition of both the oil and the food being fried. The chemical and physical changes induced in the frying fat are influenced by a number of frying parameters, with high temperature, long frying times, and metal contaminants favoring extensive decomposition of the oil. The design and type of the fryer (pan deepfat batch, or deep-fat continuous) ar...