The invention relates to a process for preparing an aromatic polycarboxylic acid by liquid phase oxidation of a di- or tri-substituted benzene or naphtalene compound, the process comprising a step of contacting the aromatic compound with an oxidant in the presence of a carboxylic acid solvent, a metal catalyst and a promoter in a reaction zone, wherein the promoter is an ionic liquid comprising an organic cation and a bromide or iodide anion. Advantages of this process include high conversion without severe corrosion problems otherwise associated with halogen-containing compounds as promoter. The process does not necessitate the use of special corrosion-resistant material or liners in the process equipment; thus offering savings on investment and maintenance costs and increasing plant reliability.The process of the invention is especially suited for production of terephthalic acid from p-xylene.