The synthesis of butyl propionate in a recirculating bioreactor in room temperature ionic liquid/supercritical carbon dioxide biphasic systems at 50 degrees C and 80 bar was studied. In these systems, alpha-alumina microporous membranes with immobilized Candida antarctica lipase B were coated with four different ionic liquids based on 1-n-alkyl-3-imidazolium cations and hexafluorophosphate and bis{(trifluoromethyl)sulfonyl}imide anions. Selectivity increased (reaching >99.5%) when room temperature ionic liquid/supercritical carbon dioxide biphasic systems were used rather than in supercritical carbon dioxide alone. To understand the behaviour of the enzyme and the mass-transfer phenomena in these biphasic systems, the reaction was also carried out in ionic liquids and in ionic liquid/hexane biphasic systems, and the ionic liquid/hexane partition coefficients of the compounds involved in the transesterification reaction were determined. It was observed that the activity in room temperature ionic liquid/supercritical carbon dioxide biphasic systems depends on the effect of the ionic liquid media on the enzyme and the diffusional limitations across the IL-layer around the biocatalyst. (C) 2007 Elsevier B.V. All rights reserved.