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Vegetable oils and animal fats can be transesterified to biodiesel for use as an alternative diesel fuel.Conversion of low cost feedstocks such as used frying oils is complicated if the oils contain large amounts of free fattyacids that will form soaps with alkaline catalysts. The soaps can prevent separation of the biodiesel from the glycerinfraction. Alternative processes are available that use an acid catalyst. The objective of this study was to investigate theeffect of process variables on acid-catalyzed transesterification. The molar ratio of alcohol, reaction temperature, catalystamount, reaction time, water content, and free fatty acids were investigated to determine the best strategy for producingbiodiesel. Food grade soybean oil was used to prepare esters using excess methanol and sulfuric acid as a catalyst. Tocompare the effect of different alcohol types on ester formation, methanol, ethanol, 2-propanol, and n-butanol werecompared. The American Oil Chemists Society Method Ca 14-56 was used to measure the biodiesels total glycerinamount as an indicator of the completeness of the reaction. It was found that acid catalysis can provide high conversionrates but much longer times are required than for alkaline catalysts. The acid catalyst also requires the concentration ofwater to be less than 0.5%, which is about the same as is required for alkaline catalysts. Water formed by theesterification of free fatty acids limited their presence in the oil to 5%.