Synthesis of oleic acid L-menthyl ester was analyzed from the viewpoint of reaction equilibrium in an oil-aqueous biphasic system with Candida rugosa lipase. When the reaction was conducted using a mixture of oleic acid/L-Menthol (1:1, mol/mol), the degree of esterification (referred to as conversion) at the equilibrium state was 80% even though the overall amount of water added in the reaction mixture was from 5 to 90 wt.%. The conversion based on L-menthol increased to 93% in the reaction conducted using oleic acid/L-Menthol (19:1, mol/mol) in the presence of 20 wt.% water, but the conversion based on oleic acid was 81% in the reaction with oleic acid/L-menthol (1:9, mol/mol). Furthermore, the contents of water in the oil phase were 0.4 and 1.4 wt.% when the reactions were conducted using 19:1 and 1:9 (mol/mol) of oleic acid/L-menthol, respectively. These results showed that excess amounts Of L-Menthol increase the solubility of water, and that the high content of water in the oil phase promotes hydrolysis Of L-menthyl ester. Because the composition in the oil phase was indicated to be important for determination of the equilibrium state, the equilibrium constant was calculated based on the contents of reactants in the oil phase. The equilibrium constant was high when the initial molar ratio of oleic acid/L-menthol was 1:1.6 to 4.2:1 compared with the cases when excess amounts of oleic acid or L-menthol were used. This phenomenon could be explained by hydrogen bonding of reactants. (c) 2006 Elsevier Inc. All rights reserved.