Abstract The anaerobic degradation of microcrystalline cellulose by thermostable cellulolytic enzyme complexes from Clostridium thermocellum JW20 (ATCC 31449) was monitored. For quantitative investigations as enzyme-coupled spectrophotometric assay has been developed. The assay allows for the evaluation of the release of cellubiose-/glucose-units from native cellulose. Kinetic studies revealed that the anaerobic breakdown of crystalline cellulose (CC) at 60°C follows Michaelis-Menten kinetics K m CC values have been determined for different aggregation states of the cellulolytic complex. The presented assay seems well suited to screen for CC-degrading enzymes of various sources, and to further explore the mechanism of CC-breakdown.