The molecular weight (or degree of polymerization, DP) of cellulose is considered to be one of the most important properties affecting the enzymatic hydrolysis of cellulose.  However, there are few studies available investigating the effects of pretreatment on biomass cellulose DP due to the difficulty in making these measurements.  In this study, size exclusion chromatography (SEC) was successfully used to measure the molecular weight distribution of cellulose samples derived from pretreated biomass.  It was necessary to prepare tetrahydrofuran-soluble cellulose tricarbanilate derivatives from pretreated corn stover to conduct SEC.  We found that in dilute acid and organosolv pretreated corn stover samples the chromatograms of the cellulose carbanilates shifted to longer retention time as pretreatment severity increased, implying that this treatment parameter caused a reduction in apparent molecular weight.  These changes in DP should significantly influence enzymatic hydrolysis of the cellulose in the pretreated biomass, because it is expected that β-1,4-exoglucanase activity is dependant on the number of available chain ends.