['There is still considerable debate regarding the actual mechanism by which a "cellulase mixture" deconstructs cellulosic materials, with accessibility to the substrate at the microscopic level being one of the major restrictions that limits fast, complete cellulose hydrolysis. In the work reported here we tried to determine the predominant mode of action, at the fiber level, of how a cellulase mixture deconstructs pretreated softwood and hardwood pulp fibers. Quantitative changes in the pulp fibers derived from different pretreated biomass substrates were monitored throughout the course of enzymatic hydrolysis to see if the dominant mechanisms involved either the fragmentation/cutting of longer fibers to shorter fibers or their "peeling/delamination/erosion," or if both cutting and peeling mechanisms occurred simultaneously.', 'Regardless of the source of biomass, the type of pretreatment and the chemical composition of the substrate, under typical hydrolysis conditions (50°C, pH\xa04.8, mixing) longer pulp fibers (fiber length >200\xa0μm) were rapidly broken down until a relatively constant fiber length of 130 to 160\xa0μm was reached. In contrast, shorter fibers with an initial average fiber length of 130 to 160\xa0μm showed no significant change in length despite their substantial hydrolysis. The fragmentation/cutting mode of deconstruction was only observed on longer fibers at early stages of hydrolysis. Although the fiber fragmentation mode of deconstruction was not greatly influenced by enzyme loading, it was significantly inhibited by glucose and was mainly observed during initial mixing of the enzyme and substrate. In contrast, significant changes in the fiber width occurred throughout the course of hydrolysis for all of the substrates, suggesting that fiber width may limit the rate and extent of cellulose hydrolysis.'] Does the enzymatic hydrolysis of pretreated pulp fibers predominantly involve `` peeling/erosion '' modes of action?