Low solvent concentration effect on substrate digestibility of ethanol organosolv pretreated sweetgum was examined. Surprisingly, lower ethanol concentration in organosolv pretreatments resulted in faster initial rates and higher 72 h hydrolysis yields in pretreated substrates. A strong correlation (r2 = 0.96) between pretreatment combined severity factor and residual xylan/glucan ratio was observed. The residual xylan/glucan ratio was associated with the initial hydrolysis rate closely. Furthermore, it was found that preserving extractable lignin in the pretreated substrates could improve enzymatic hydrolysis yield by 33%. This has an important implication in reducing the pretreatment and enzyme cost, because the typical solvent washing after pretreatment could be eliminated and preserving extractable lignin could reduce enzyme loading. Finally, we observed that xylan removal by xylanase could improve the initial rate by 53% and increase the 72 h hydrolysis yield by 21%. The extractable lignin precipitation on pretreated substrates increased the 72 h hydrolysis yield by 10%.