Ionic liquids are the basis of a promising technology for the dissolution and fractionation of lignocellulosic materials. The use of a miscible organic cosolvent as additive may improve the physical properties of the dissolving fluid, as compared to those of the ionic liquid alone. Alternatively, this organic substance added in larger amounts can act as antisolvent for the regeneration of the nonvolatile lignocellulose fractions dissolved. In this work, mixtures of methanol and 1-ethyl-3-methylimidazolium acetate (an archetypal ionic liquid for dissolution of lignocelluloses) were characterized from a thermal and physical point of view. Their ability to dissolve representative standards of the lignocellulose biopolymers (cellulose, hemicellulose, lignin) was assessed. Very different solubilities of the three biopolymer standards were observed as a function of composition of the solvent, suggesting the possibility of a selective dissolution procedure to separate mixtures of the disengaged polymers. Precipitation tests showed that fractionated recovery of the dissolved biopolymers from the ionic liquid was not possible by direct addition of moderate amounts of methanol, due to formation of gel-like phases.