In an earlier binding study conducted in our laboratory using Thermobifida fusca cellulases Ce16B, Ce19A, and Ce15A (formally Thermomonospora fusca E-3, E-4, and E-5), it was observed that binding capacities for these three cellulases were 18-30 times higher on BMCC than on Avicel. These results stimulated an interest in how the difference in accessibility between the two cellulosic substrates would affect synergism observed with cellulase mixtures. To explore the impact of substrate accessibility on the extent of conversion and synergism, three binary T. fusca cellulase mixtures were tested over a range of cellulase ratios and total molar cellulase concentrations on Avicel and BMCC. Higher extents of conversion were observed for BMCC due to the higher enzyme to substrate ratio resulting from the higher binding. The processive endoglucanase, Ce19A, had four times the extent of conversion of the endocellulase Ce15A, while the exocellulase Ce16B had three times the extent of conversion of Ce15A. Approximately 500 nmol/g of the Ce19A+Ce16B mixture was needed to obtain 80% conversion, while the Ce16B+Ce15A and Ce19A+Ce15A mixtures required 1500 and 1250 nmol/g, respectively, to obtain 80% conversion. Thus, it appears that the more accessible structure of BMCC, as reflected by its binding capacity, results in relative higher processive activity.