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Abstract: Glucomannans from Bletilla striata (bletillan) were used as excipient for controlled deliveries of drugs, genes and tissue engineering. In the present study, a controlled acetylation method was developed to improve water solubility of bletillan 70 (BT) firstly, by reacting with acetic anhydride (AA) in N,N-dimethylformamide solvent. The preparation parameters, such as reaction temperature, reaction time and molar ratio of BT/AA, were optimized based on degrees of acetyl group in addition. IR and 1H NMR spectra were applied to elucidate the reaction process and substitution pattern, which indicated that the acetylation took place at C-6 and C-2 of the hexose units in a ratio of 2:1, with DS up to 0.83. Relative viscosity analysis revealed that the resulted products had improved water solubilities. This novel method is simple, economic and easily controlled. [Copyright &y& Elsevier]