The effects of additives on hydrolysis of microcrystalline cellulose (AvicelPH101) were examined using commercial cellulose-degrading enzymes (Celluclast 1.5 L and Novozyme 188). Polyethylene glycol 4000 (PEG4000) was the most effective additive tested. When PEG4000 was added at 0.05 g/g glucan, the conversion of Avicel PH101 increased 91% (from 41.1% to 78.9%). The cellulase activity of Celluclast 1.5 L increased 27.5% with PEG4000 addition. A positive effect on enzyme stabilities of Celluclast 1.5 L and Novozyme 188 also occurred with PEG4000 addition. During hydrolysis process, significant changes in free protein concentration and cellulase activity were observed on Avicel PH101. More than 90% of the original enzyme activity remained in the solution after 48 h hydrolysis. Thus, PEG4000 addition is an efficient method to enhance digestibility of cellulosic materials and make enzyme recovery possible and valuable. This provides an opportunity of decreasing the operational cost of the hydrolysis process. (C) 2010 Elsevier Ltd. All rights reserved.