Accessible internal pore surface is a major factor in defining the rate and extent of enzymatic hydrolysis of cellulosic biomass. Steric hindrance within the pore structure can be a major factor limiting the accessibility of cell-wall-degrading-enzymes (CWDEs) to reactive surface area. My research investigated the fundamental mechanisms involved in limiting CWDEs accessibility to reactive surface area from three aspects. First, a high resolution microscopy platform was developed to gain insights into the diffusion hindrance that CWDEs may encounter in crowded environments inside the biomass porous space. The diffusion activity of fluorescently-labeled non-bound dextran probes in pore space of filter paper particles was observed using a high resolution CLSM microscopy platform. A pore grouping diffusion model was developed and modeling results show that 75% of the accessible pore volume is available for fast diffusion without any significant pore hindrance. Second, a novel solute exclusion system was developed to measure specific pore volume and specific surface areas for raw and pretreated mixedhardwood (MHW) and switchgrass (SG). Replicate measurements of probe concentrations consistently yielded coefficient of variance of less than 1.5%. Particle size reduction had a smaller influence on the specific pore volume distribution of raw biomass. Pore surface area accessible to 5.1 nm probe increased 4-5 folds for pretreated MHW and SG. A pore size change mechanism was proposed that could explain the influence of size reduction and pretreatment on pore volume measurements. Third, pore size distribution changes during the hydrolysis process were investigated to elucidate the intrinsic hydrolysis mechanisms of CWDEs. Pretreated SG reached the plateau after 12 hr of hydrolysis and showed a faster initial hydrolysis rate than pretreated MHW. Most of the CWDEs have been removed by protease from the system as shown by SDS-PAGE gel and Bradford assay analysis. Results showed a 30% initial declining of specific pore volume for both MHW and SG after the first 2 hours of hydrolysis. The resulting accessible reaction surface drop can directly contribute to the decreasing in hydrolysis rate.