AbstractCorn bran is a by-product from corn starch processing. This work examined the effects of changing substrate particle size on enzymatic hydrolysis of both raw and pretreated destarched corn bran. The biomass composition of the corn bran varied between particle size fractions: The largest particles ([1000;710]µm) were richer in cellulose and in (arabino) xylan with a relatively low degree of arabinofuranosyl substitutions, whereas the smaller particles ([250;150]µm) contained less cellulose, but arabinoxylan with higher arabinofuranosyl substitution (higher A:X ratio). Enzymatic hydrolysis yields improved with decreasing substrate particle size, particularly for the raw corn bran. The increased enzymatic yields obtained with decreasing substrate particle sizes were related to the increased substrate surface area but also to the biomass composition. Theoretical estimations of enzymatic reaction efficiency supported that biomass composition affected the enzymatic reaction yields and provided new insight into the impact of substrate particle size on enzymatic biomass hydrolysis.