Biodiesel was produced by ethanolysis of pure castor and soybean oils, and mixtures thereof, using potassium hydroxide as catalyst. The yields and selectivities of these reactions were evaluated. The results revealed that there was no appreciable substrate preference when the vegetable oils were transesterified in admixture. However, higher reaction yields and increased efficiencies of the purification process were directly correlated with the proportion of soybean oil present in the reaction mixture. (C) 2010 Elsevier Ltd. All rights reserved.