The influence of water activity on the synthesis of butyl butyrate by transesterification reaction (alcoholysis) using lipases from different sources was studied. A direct relationship between water activity (a(w)) and reaction rate was observed. The lipases exhibited an independent water activity profile for maximum transesterification rate. Maximum esterification yield (92%) was obtained with Candida rugosa lipase when compared to the yields from other lipases (Rhizopus oryzae, 45%; Mucor javanicus, 84%; Aspergillus niger, 56% and Penicillium roqueforti, 67%). At higher water activity (a(w): 0.96) the reaction equilibrium favoured the hydrolysis of ethyl butyrate and the equilibrium was shifted to synthetic mode (butyl butyrate) when operated at low water activity (0.33) levels. Optimum water activity for obtaining maximum yield varied with the concentration of substrate used for the reaction. When substrate concentrations of 50 and 100 mM were used the maximum yields were obtained at a(w) of 0.33. When substrate concentrations of 150 and 200 mM were used maximum yields were obtained at a(w) of 0.54. At higher water activity levels (a(w) 0.96) almost all the enzymes showed lower rates which could be due to protein aggregation. (C) 2002 Elsevier Science Ltd. All rights reserved.