An environmentally-friendly method to separate cellulose and hemicelluloses from lignin in recalcitrant biomass for subsequent conversion is desirable to reduce greenhouse gas generation. Easily-prepared, deep eutectic solvents (DESs) have low volatility, wide liquid range, non-flammability, nontoxicity, biocompatibility, and biodegradability. This study shows the DESs (formic acid: choline chloride, lactic acid: choline chloride, acetic acid: choline chloride, lactic acid: betaine, and lactic acid: proline) to be capable of preferentially dissolving lignin at 60 degrees C. Thermogravimetric analysis show DES to be stable at typical biomass processing temperatures. Pretreating loblolly pine in one DES increased glucose yield after enzymatic hydrolysis to more than seven times that of raw or glycerol-pretreated pine. The density of DES-pretreated biomass was found to be 40% higher than the untreated pine's density. (C) 2017 Elsevier Ltd. All rights reserved.