The saccharification of microcrystalline cellulose by the cellulase complex of Trichoderma viride has been investigated using a membrane reactor. Under the experimental conditions adopted it was possible to evaluate the contributions to the overall process of the sequentially acting glucanases and β-glucosidase. The endo- and exoglucanases produce mainly cellobiose, which is converted into glucose by the β-glucosidase. It was found that both reactions obey Michaelian kinetics. Both cellobiose and glucose inhibit the cellulase complex. These effects could be controlled by working at higher space velocities, provided that polarisation of the membrane was avoided. The results obtained suggest the use of membrane reactors as a convenient method for studying the kinetics of complex breakdown processes such as cellulose hydrolysis.