Lignocellulosic biomass materials are fractionated by dissolving at least a portion of the lignocellulosic material in an ionic liquid and separating the resulting mixture into a cellulose-enriched fraction and/or a lignin-enriched fraction. The cellulose-enriched fraction contains about 10 percentage points more glucose than the weight percent glucose contained in the original lignocellulosic biomass material. In certain embodiments, the lignin-enriched fraction includes a lignin content that is 5 percentage points greater than the weight percent lignin present in the original lignocellulosic material. The techniques used to isolate the cellulose-enriched fractions and/or the lignin-enriched fractions from the ionic liquid mixture do not employ environmentally unfriendly solvents.