Immobilized Candida antarctica lipase B (CALB) was successfully used as catalyst to synthesize butyl butyrate from butyl vinyl ester and 1-butanol in supercritical carbon dioxide (scCO(2)) with excellent results. The catalytic behaviour of the enzyme immobilized on an acrylic support has been studied in a stirred tank reactor, showing that a decrease in both the water content and the scCO(2) density enhanced the synthetic activity and selectivity (> 99.0%). Then, ceramic membranes were coated with hydrophilic polymers, and then used to covalently attach CALB. These active membranes were applied for continuous butyl butyrate synthesis in a cross-flow reactor with different organic solvents and supercritical conditions, as reaction media. A clear enhancement in the synthetic activity and selectivity was observed with the decrease in fluid density for both liquids and scCO(2) media. However, all supercritical conditions assayed enhanced up 84-folds respect to the organic solvents the synthetic activity of the lipase-membrane derivative. For the best supercritical conditions (60degreesC, 8 MPa), the enzymatic membrane was assayed by repetitive operational cycles of 6 h/day, showing a 360 cycles half-life time in their synthetic activity. (C) 2003 Elsevier B.V. All rights reserved.