Enzymatic acetylation of carboxymethyl cellulose (CMC) with vinyl acetate by lipase A12 from Aspergillus niger (A. niger) in buffer was used as a model reaction for investigating lipase-catalyzed ester synthesis in aqueous solution. An increase in CMC concentration (1-15 wt.%) enhanced the reaction yield via thermodynamically favoring the CMC ester synthetic reaction in aqueous medium. The use of vinyl acetate as activated acetyl donor kinetically drove the forward transesterification over product hydrolysis, but partially inhibited enzyme activity when used at large amounts ( > 5 mL/g CMC). The increase in enzyme amount initially improved the reaction markedly, but less efficiently at larger enzyme amounts ( > 1000 lipase activity units/g CMC) due to the substrate diffusion limitation. The difference in reaction course demonstrated that the lipase-catalyzed transesterification was under kinetic control, while the non-enzymatic reaction under thermodynamic control. The acetylation yield of the lipase-catalyzed reaction was 5.6 times higher than that of the non-enzymatic reaction. This increase in reaction yield was attributed partly (by 19%) to the side cellulase activity of the lipase due to its substrate degradation role, but largely (81%) to the lipase activity by virtue of its transesterification catalysis function. The A. niger lipase was identified as a hydrolase capable of catalyzing ester synthesis in aqueous solution. (C) 2004 Elsevier Inc. All rights reserved.