This paper deals with transesterification of soybean oil with ethanol in the presence of Novozym 435 (lipase from Candida antarctica) catalyst. H-1 NMR was employed to monitor the conversion of soybean oil to fatty acid ethyl esters in a solvent-free medium. Reaction conditions such as enzyme/substrate ratio, temperature and alcohol to oil molar ratio were studied and the highest yields were found at 5 g of enzyme per 100 g of oil, 37 degrees C, and 3:1 respectively. Stepwise addition of ethanol was also performed, and compared with that of methanol, in order to assess the reduction of inhibition that this method causes on the enzymatic activity. (C) 2013 Elsevier Ltd. All rights reserved.