The effect of the degree of acetylation on the enzymatic digestibility of acetylated xylans has been investigated. Oatspelts xylans were reacetylated to degrees of 0.26 to 1.67 moles acetyl groups per mole of anhydroxylose units. These acetylated samples were then used to study the effect of acetylation on the xylanase (EC 3.2.1.8) and acetyl esterase (EC 3.1.1.6) activities of a commercialTrichoderma reeseicellulase preparation. The enzymatic digestibility was dramatically affected by the degree of acetylation. The onset of resistance is similar for both the xylanase and acetyl esterase enzymes, and both enzymes were completely inhibited by a degree of acetylation of 1.5 moles acetyl groups per mole anhydroxylose units at all enzyme loadings.