Cellulase reaction mechanism was investigated with the use of following pure cellulosic substrates: Microcrystalline cellulose (Avicel), α-cellulose (Sigma), filter paper, cotton, and non-crystalline cellulose (NCC). NCC is amorphous cellulose prepared in our laboratory by treatment with concentrated sulfuric acid. When hydrolyzed with cellulase, NCC produces significant amount of cello-oligosaccharides (COS) as reaction intermediates along with glucose and cellobiose. The COS produced by cellulase were categorized into two different moieties based upon their degree of polymerization (DP): low DP (less than 7) COS (LD-COS) and high DP COS (HD-COS). Endo-glucanase (Endo-G) reacts rapidly on the NCC reducing its DP to 30–60, after which the Endo-G reaction with NCC ceases. HD-COS is produced from NCC by the action of Endo-G, whereas LD-COS is produced by exo-glucanase (Exo-G). β-Glucosidase (β-G) hydrolyzes LD-COS to produce cellobiose, but it does not hydrolyze HD-COS. DP of NCC affects the action of Exo-G in such a way that the overall yield is high for high DP NCC. This is in line with previous findings that substrate-recognition by Exo-G requires binding on β-glucan chain with DP of 10 for the hydrolysis to take place. The individual cellulose chain residues within solid having DP less than 10 therefore remain unreacted. The percentage of the unreacted portion would be lower for high DP NCC, which results high overall conversion. The surface area and the number of reactive sites on the substrate facilitate adsorption of enzyme therefore the initial rate of the hydrolysis. The overall extent of conversion of cellulose, however, is controlled primarily by its inherent characteristics such as DP and crystallinity. Biotechnol. Bioeng. 2009;102: 1570–1581. © 2008 Wiley Periodicals, Inc.