The dilute acid (0.05 M H2SO4) hydrolysis at 175 degrees C of samples comprised of varying fractions of crystalline (alpha-form) and amorphous cellulose was studied. The amorphous content, based on XRD and CP/MAS NMR, and the product (glucose) yield, based on HPLC, increased by as much as a factor of 3 upon ball milling. These results are interpreted in terms of a model involving mechanical disruption of crystallinity by breaking hydrogen bonds in alpha-cellulose, opening up the structure, and making more beta-1,4 glycosidic bonds readily accessible to the dilute acid. However, in parallel with hydrolysis to form liquid-phase products, there are reactions of amorphous cellulose that form solid degradation products.