An ionic liquid, 1-ethyl-3-methylimidazolium acetate (EmimOAc) better catalyzed the acetylation of the aliphatic hydroxyl groups of lignin than the phenolic hydroxyl groups using isopropenyl acetate as an acetyl donor. However, in situ (HNMR)-H-1 analyses performed on model compounds revealed that EmimOAc more effectively promoted the acetylation of phenol than 2-phenethyl alcohol. EmimOAc was also found to selectively catalyze the deacetylation of aromatic acetate, and thus the aliphatic hydroxyl groups of lignin were preferentially acetylated.