The equilibrium constants for the formation of fatty acid butyl esters by the lipase-catalyzed condensation were estimated in nitriles, tertiary alcohols and their mixtures. The equilibrium constant for the synthesis of butyl decanoate did not depend on the alkyl chain length of the nitriles or alcohols, but significantly depended on the type of solvent polar group. The constant in the mixture of a nitrile and a tertiary alcohol increased as the molar fraction of the nitrile in the mixture increased. The IR spectra of the C=O bond of decanoic acid and butyl decanoate were measured in various solvents. The measurements indicated that the interaction between the decanoic acid and t-butyl alcohol was stronger than that between butyl decanoate and the alcohol. The interaction would be a reason for the dependence of the equilibrium constant on the fraction of nitrile in a mixed solvent. (C) 2003 Elsevier B.V. All rights reserved.