The complex [Sn(3-hydroxy-2-methyl-4-pyrone)(2)(H2O)(2)] immobilized in 1-n-butyl-3-methylimidazolium tetrachloro-indate (BMI.InCl4) ionic liquid forms an effective biphasic catalytic system for the production of biodiesel from the alcoholysis of soybean oil. ESI-MS experiments during the transesterification reaction indicate that the reaction may proceed through the formation of a cationic species formed through the substitution of one pyrone ligand by one alcohol molecule, followed by coordination of the carboxylate compound to tin. The adduct thus formed undergoes transesterification with liberation of the catalytic active species. (c) 2007 Elsevier Inc. All rights reserved.