Equilibrium binding of Thermomonospora fusca E-3, E-4 and E-5, the E-3 binding domain (CBDE3), and Trichoderma reesei CBHI on Avicel PH102 and bacterial microcrystalline cellulose (BMCC) was studied. The maximum adsorption levels, E-b,E-m, for all four cellulases and the binding domain were 9-30 times higher on BMCC than on Avicel. The association constants for the individual cellulases were dependent upon the substrate; however no obvious patterns were noted. A comparison of the T. fusca EbmS showed a decreasing power function relationship between molecular weight and maximum adsorption levels. This was particularly true for the cellulases binding on Avicel. The T. fusca binding results strongly suggest that binding capacity is a function of the cellulase size and the pore structure of the cellulose.