The digestibility of solids following pretreatments by leading technologies of ammonia fiber expansion, ammonia recycle percolation, controlled pH, dilute acid, lime, and sulfur dioxide were evaluated at different cellulase loadings for two distinct types of cellulosic biomass from controlled sources: an agricultural residue, corn stover, and a woody biomass, poplar. Furthermore, in order to understand the differences in their digestion performance, these solids were characterized hypothesizing that enzyme adsorption on cellulose and its effectiveness are the two primary factors controlling enzymatic saccharification. The accessibility of cellulose and lignin to cellulase (maximum adsorption capacity, σ) was determined, and the effect of selected substrate features on cellulase adsorption on cellulose and its effectiveness were evaluated.