The sorption of Thermomonospora fusca E-3 and E-5 and Trichoderma reesei CBHI cellulases on bacterial microcrystalline cellulose (BMCC) was investigated to measure binding reversibility and surface exchange. The adsorption and desorption curves for CBHI were comparable; this suggests a totally reversible binding process. By contrast, the desorption isotherms of E-3 and E-5 did not retrace their respective adsorption curves; instead, hysteresis loops were formed. The calculated percent reversibilities for E-3, E-5, and CBHI were 73 +/- 7%, 80 +/- 7%, and 95 +/- 7%, respectively. Surface exchange studies indicated that the adsorbed and free cellulases were exchanging at the cellulose surface. Despite this confirmation of molecular exchange, the extent of exchange did not reach the level that would be predicted if the preadsorbed cellulase were to equally redistribute between the free and bound states. The percent of exchange for E-3 and E-5 decreased with increasing initial total cellulase concentration (E-t) while that for CBHI showed no dependence on E-t.