Optically active cyanohydrin esters such as (S)-alpha-cyano-3-phenoxybenzyl (CPB) acetate can he obtained with enzymatic transesterification of racemic cyanohydrins in organic media. The influence of internal diffusion limitation on transesterification of CPB alcohol with vinyl acetate catalyzed by immobilized lipase was studied. Internal diffusion limitation could not be ignored since the immobilized lipase granule was not small enough. In order to express the effect of internal diffusion limitation quantitatively. a method was proposed which calculates the effective diffusion coefficient (D-e) first and then calculates apparent Thiele modulus (Phi) and finally obtains internal diffusion effectiveness factor, eta value (0.55). D-e was calculated to be 3.1 x 10(-10) m(2) s(-1) by supposing that the immobilized lipase was a sphere granule and the diffusion only existed in one dimension. Phi was calculated to be 2.2 by presuming that transport of substrate through the catalyst could be described by Fick's law. Using King-Altman method, a kinetic model of the transesterification of CPB alcohol with vinyl acetate in organic media in the presence of internal diffusion limitation was proposed based on ping-pong bi-bi mechanism. The relative error of the model was 11.18%. A practicable method to evaluate eta value and relatively simple model for such kind of reaction system was proposed in this paper. (C) 2004 Elsevier Inc. All rights reserved.