A comparative study was made of immobilized Burkholderia cepacia lipase (PSL-C)-catalyzed acylation of lily polysaccharide (LP) with vinyl acetate in organic solvents, ionic liquids (ILs) and IL-containing systems. The degree of substitution (DS) of the modified LP was used to evaluate the extent of acylation and thus enzymatic activity. In this manner, an eco-friendly solvent, 2-methyltetrahydrofuran (MeTHF), was found to be the most suitable organic reaction medium. However, compared to MeTHF, enhanced enzyme activity was observed when 1-butyl-3-methylimidazolium tetrafluorobrate ([C(4)MIm][BF4]) was used as the solvent. To further enhance the DS of the modified LP product, co-solvent mixtures of [C(4)MIm][BF4] and MeTHF were investigated. Among the various MeTHF-[C(4)MIm][BF4] systems examined, 20% (v/v) MeTHF-[C(4)MIm][BF4] produced the highest DS. In this reaction medium, the optimal water activity, reaction temperature and time were 0.33, 55 degrees C and 18h, respectively, producing a product DS as high as 0.67. The PSL-C enzyme exhibited a much higher stability in the IL-containing system. Additionally, PSL-C-catalyzed acylation of LP was highly regioselective, causing acylation of only C6-OH. (C) 2013 Elsevier Ltd. All rights reserved.