Regioselective acylation of konjac glucomannan (KGM) with vinyl acetate in a solvent-free system was successfully performed using Novozym 435 as a biocatalyst. The degree of substitution (DS) of the modified KGM was used to evaluate the extent of acylation. The influence of various factors, such as water activity (a(w)), reaction temperature, shaking rate, enzyme dosage, and the molecular weight of KGM, on the reaction was examined. The water activity of the reaction system played a key role in the acylation of KGM. The optimum water activity, reaction temperature, shaking rate, and enzyme dosage were 0.84, 50 degrees C, 200 rpm and 400 U/mL, respectively. It has also been found that the DS of modified KGM sample decreases with increase in KGM molecular weight. Additionally, the acylation of KGM was shown to be regioselective, with acylation occurring at the C6-OH. (c) 2006 Wiley Periodicals, Inc.