Cellulose is converted to glucose in a two stage process in which cellobiose is produced from a cellulosic feedstock under the influence of Trichoderma reesei in a first stage and cellobiose from the first stage is converted to glucose in a second stage by the action of purified cellobiase derived from Aspergillus terreus. Cellobiase from A. terreus is purified by contacting a crude aqueous extract of the cellobiase with an ion exchange resin and an anion exchange resin. The purified cellobiase may be immobilized on a suitable substrate.