Heterogeneous cellulose accessibility is an important substrate characteristic, but all methods for determining cellulose accessibility to the large-size cellulase molecule have some limitations. Characterization of cellulose accessibility to cellulase (CAC) is vital for better understanding of the enzymatic cellulose hydrolysis mechanism (Zhang and Lynd, Biotechnol. Bioeng. 2004, 88, 797-824; 2006, 94, 888-898). Quantitative determination of cellulose accessibility to cellulase (m2/g of cellulose) was established based on the Langmuir adsorption of the fusion protein containing a cellulose-binding module (CBM) and a green fluorescent protein (GFP). One molecule of the recombinant fusion protein occupied 21.2 cellobiose lattices on the 110 face of bacterial cellulose nanofibers. The CAC values of several cellulosic materials -- regenerated amorphous cellulose (RAC), bacterial microcrystalline cellulose (BMCC), Whatman No. 1 filter paper, fibrous cellulose powder (CF1), and microcrystalline cellulose (Avicel) -- were 41.9, 33.5, 9.76, 4.53, and 2.38 m2/g, respectively. The CAC value of amorphous cellulose made from Avicel was 17.6-fold larger than that of crystalline cellulose - Avicel. Avicel enzymatic hydrolysis proceeded with a transition from substrate excess to substrate limited. The declining hydrolysis rates over conversion are mainly attributed to a combination of substrate consumption and a decrease in substrate reactivity. Declining heterogeneous cellulose reactivity is significantly attributed to a loss of CAC where the easily hydrolyzed cellulose fraction is digested first.