Ground mixed hardwood samples were hydrolyzed with cellulase before and after a short-term pretreatment with sulfuric acid in a plug flow reactor at 180, 200 and 220°C. SEM observations of the Substrates indicate that the enhancement of enzymatic hydrolysis can be attributed mainly to the increment in surface area. Lignin has an inhibiting effect on the enzyme äs it impedes physical contact between cellulose and enzyme. After delignification and alkali extraction alpha-cellulose can be converted by the enzyme almost completely into monomeric sugars.