Product inhibition by cellobiose decreases the rate of enzymatic cellulose degradation. The optimal reaction conditions for two Emericella (Aspergillus) nidulans-derived cellobiohydrolases I and II produced in Pichia pastoris were identified as CBHI: 52 A degrees C, pH 4.5-6.5, and CBHII: 46 A degrees C, pH 4.8. The optimum in a mixture of the two was 50 A degrees C, pH 4.9. An almost fourfold increase in enzymatic hydrolysis yield was achieved with intermittent product removal of cellobiose with membrane filtration (2 kDa cut-off): The conversion of cotton cellulose after 72 h was similar to 19 % by weight, whereas the conversion in the parallel batch reaction was only similar to 5 % by weight. Also, a synergistic effect, achieving similar to 27 % substrate conversion, was obtained by addition of endo-1,4-beta-d-glucanase. The synergistic effect was only obtained with product removal. By using pure, monoactive enzymes, the work illustrates the profound gains achievable by intermittent product removal during cellulose hydrolysis.