The enzymatic saccharification of a model cellulosic substrate, Avicel PH- 101, using an ionic liquid ( IL), 1- ethyl- 3- methylimidazolium diethylphosphate, was explored. After mixing the IL solution of cellulose with different volumes of 10 mM citrate buffer ( pH 5.0), cellulase was directly added to the aqueous- IL mixture at 40 degrees C. When the volume of IL to water was greater than 3: 2, little cellulase activity was observed. However, decreasing the volume ratio markedly enhanced enzymatic activity: an IL to water ratio of 1: 4 ( v/ v) resulted in over 70% of the starting amount of cellulose ( 10 mg/ ml) being converted to glucose and cellobiose.