Soybean oil was transesterified using various mixtures of methanol and ethanol at a constant molar ratio of alcohol to oil of 12:1 in the presence of 1 wt% potassium hydroxide catalyst at 30 degrees C for 60 min. The effect of mixtures of methanol and ethanol on percentage yields and fuel properties of soybean oil methyl and ethyl esters (SME/SEE) was studied. The formation of methyl esters was faster in comparison to ethyl esters, as evidenced by a molar ratio of methyl to ethyl esters of 1.7:1 in the case of a 1:1 mixture of methanol and ethanol. High yields (94.6-97.9%) were obtained for all molar ratios of reactants studied. The resultant SME/SEE mixtures prepared from a methanol to ethanol molar ratio of 1:2 or greater exhibited enhanced low temperature properties and oxidative stability, as well as superior lubricity in comparison to neat SME. All other ratios that contained a higher percentage of methanol displayed similar behavior to pure SME. These results indicate that the fuel properties of soybean oil-based biodiesel can be improved by substituting a portion of the methanol reagent with ethanol during transesterification, albeit at higher production cost as a result of the higher price of ethanol versus methanol. (C) 2009 Elsevier Ltd. All rights reserved