Continuous production with in situ separation of fatty acid ester in ionic liquid (IL) was investigated through lipase-catalyzed transesterification of butyl alcohol and vinyl laurate in 1-methyl-3-octylimidazolium trifluoromethanesulfonate ([Omim][TfO]) as a model reaction. Since the ester product is immiscible with IL, continuous separation of the product from reaction media can be easily achieved. The optimized reaction conditions for lipase-catalyzed transesterification in [Omim][TfO] were experimentally studied in batch operation. To combine continuous reaction operation with in situ separation exploiting immiscibility, a double-layer type continuous stirred tank reactor was developed. By using this reactor, the efficiency of in situ separation of product from IL and the operational stability were tested at various flow rates (0.375-0.75 mL/h) under optimized reaction conditions determined in batch operation (1.2:1 molar ratio of butyl alcohol to vinyl laurate and 10 mg of Novozym 435 in 0.5 mL [Omim][TfO]). Results proved that continuous ester production in an integrated production/separation process was successful without any loss of IL and was stable for at least 5 days after a steady-state was reached. Our investigations demonstrate that this continuous production integrated with in situ separation system provides a promising basis for the development of environment-friendly processes for practical production and separation of compounds in ILs. (C) 2010 Elsevier Inc. All rights reserved.