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.