The demethylation of lignin in ionic liquids (ILs) was investigated by using pure lignin model monomers and dimers together with dioxane-isolated lignins from poplar, miscanthus, and maize. Different methylimidazolium ILs were compared and the samples were treated with two different heating processes: microwave irradiation and conventional heating in a sealed tube. The conversion yield and influence of the treatment on the lignin structure were assessed by P NMR spectroscopy, size-exclusion chromatography, and thioacidolysis. The acidic methylimidazolium IL [HMIM]Br was shown to be an effective combination of solvent and reagent for the demethylation and depolymerization of lignin. The relatively mild reaction conditions, the clean work-up, and the ability to reuse the IL makes the described procedure an attractive and new green method for the conversion of lignin to produce phenol-rich lignin oligomers.