We present a Raman spectroscopic study of the conversion of soybean oil into biodiesel by methanol using the lipase B enzyme pseudomonas cepacia as the bio-catalyst and 1-alkyl-3-methylimidazolium ionic liquids (C(n)C(1)Im) as co-solvents, where the alkyl chain length is varied from ethyl (n = 2) to decyl (n = 10). We have limited this study to a low reaction temperature, 40 degrees C, and a low enzyme concentration to investigate the possibility of producing biodiesel with low energy and cost demands. We also demonstrate that Raman spectroscopy is a powerful and straightforward method to estimate the yield of the transesterification reaction, by analysis of the characteristic C=0 stretching mode found in the range similar to 1730-1750 cm(-1). Our results indicate that both reaction yield and reaction rate increase with the chain length, but also that for chains longer than decyl further improvements are marginal. We discuss these results from the viewpoint of local interactions and local structure of the investigated ionic liquids. (C) 2015 Elsevier Ltd. All rights reserved.