Enzyme hydrolysis of pretreated cellulosic materials slows as the concentration of solid biomass material increases. even though the ratio of enzyme to cellulose is kept constant. This form of inhibition is distinct from substrate and product inhibition, and has been noted for lignocellulosic materials including wood, corn stover, switch grass, and corn wet cake at solids concentrations greater than 10 g/L Identification of enzyme inhibitors and moderation of their effects is of considerable practical importance since favorable ethanol production economics require that at least 200 g/L of cellulosic substrates be used to enable monosaccharide concentrations of 100 g/L, which result in ethanol titers of 50 g/L Below about 45 g/L ethanol, distillation becomes energy inefficient. This work confirms that the phenols: vanillin, syringaldehyde, trans-cinnamic acid, and hydroxybenzoic acid, inhibit cellulose hydrolysis in wet cake by endo- and exo-cellulases, and cellobiose hydrolysis by beta-glucosidase. A ratio of 4 mg of vanillin to I mg protein (0.5 FPU) reduces the rate of cellulose hydrolysis by 50%. beta-Glucosidases from Trichoderma reesei and Aspergillus niger are less susceptible to inhibition and require about 10x and 100x higher concentrations of phenols for the same levels of inhibition. Phenols introduced with pretreated cellulose must be removed to maximize enzyme activity. (C) 2010 Elsevier Inc. All rights reserved.