Ionic liquid (IL) treatment of lignin is a promising strategy to producing nonpetroleum-based polyols for diverse polyurethane (PU) applications. Herein, IL 1-butyl-3-methylimidazolium ([BMIM]Br) was employed to treat organosolv hardwood (OS-HW) and softwood (OS-SW) lignins, as well as kraft softwood (Kr-SW) lignin. Phosphorus nuclear magnetic resonance spectroscopy (31P NMR) and gel permeation chromatography (GPC) analyses revealed the highest impacts of [BMIM]Br treatment on OS-HW lignin, where [BMIM]Br-treated OS-HW lignin showed 40% increase in hydroxyl content and 20% decrease in molecular weight. By contrast, compositional changes to OS-SW and Kr-SW after [BMIM]Br treatment were not detected. [BMIM]Br treatment of OS-HW lignin increased its reactivity with isocyanate by 42%, and X-ray photoelectron spectroscopy (XPS) analysis of [BMIM]Br-treated OS-HW lignin after washing confirmed the potential to recover the IL. Together, these results demonstrate the efficacy of IL treatment in improving the applicability of certain lignins as polyol alternatives.