Lactate esters are increasingly used in food, cosmetic and pharmaceutical formulations due to their hygroscopic, emulsifying and exfoliating properties. The lipase-catalysed synthesis of butyl lactate by transesterification was investigated, in order to provide some basic design criteria. Novozyme SP435 (from Candida antarctica) was found to be the most effective biocatalyst for the reaction of interest. The optimum conditions for catalytic activity were determined in terms of temperature (about 70degreesC) and thermodynamic water activity (a(w) = 0.060). The effect of the substrate concentration was characterised, as well. The enzyme stability appeared to be critically affected by the reaction medium composition. In particular, increases in the concentration of the lactate esters (both substrate and products) resulted in faster inactivation kinetics, suggesting that the operation in solvent-free medium requires a suitable design of the operating conditions to meet the lipase stability requirements. Improved lipase stability and higher yield were obtained at reduced values of water thermodynamic activity, though the complete dehydration of the enzyme lowered the catalytic activity. (C) 2003 Elsevier Inc. All rights reserved.