Typically, biodiesel is produced using vegetable oil and methanol as raw materials, and sodium methoxide as catalyst, whereupon the obtained product is composed of methyl esters. However, the use of ethyl esters as biodiesel presents many advantages compared to the methyl esters. In this work, the transesterification with ethanol to produce ethyl esters using sodium ethoxide as catalyst was studied. The effect of temperature and alcohol and catalyst concentration on the reaction conversion was investigated, in order to optimize these parameters while also meeting the quality standards. It was found that the optimal reaction conditions are: 1.6 wt.% sodium ethoxide, 25 v/v% ethanol and 55 degrees C, which allow obtaining a biodiesel composed 100% by ethyl esters that complies with the international quality standards. On the other hand, it was observed that methanolysis reactions are faster than the ethanolysis and sodium methoxide catalyst is more active than the corresponding ethoxide, due to the higher steric hindrance of the ethoxi-radical and to the more intense saponification of sodium ethoxide in ethanol medium. (C) 2014 Elsevier B.V. All rights reserved.