Enzyme accessibility has been proposed as a limiting factor in the enzymatic conversion of the cellulose in biomass to glucose. Prior work has shown a strong correlation between porosity, measured as the change in the volume of pores accessible to a cellulase-sized molecule, and the initial digestibility of biomass pretreated by various methods. The goal of this work was to determine if porosity was one of the factors governing the overall enzymatic digestibility of the cellulose in dilute acid pretreated biomass. The porosity of wet pretreated corn stover was determined using the methods of solute exclusion and 1H nuclear magnetic resonance (NMR) thermoporometry. The solute exclusion method identified differences in the accessible pore volume of the pretreated samples compared to untreated corn stover; however, only very small differences in porosity were observed among samples pretreated with a range of severities, giving ethanol yields from 70 to 96%. No correlation was found between the volume accessible to an enzyme-sized molecule (diameter estimated to be 51 A) and the digestibility of the cellulose in dilute acid pretreated corn stover. 1H NMR thermoporometry was used to measure the amount of water in pores ranging from 20 to 200 A. As was the case for the solute exclusion method, a difference was observed in the pore volume of untreated and acid pretreated corn stover, but no significant differences in pore volume were measured for the different pretreated samples.