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SO3H-based acidic ionic liquids were used as Brønsted acid catalysts for synthesis of diphenolic acid (DPA) from the condensation of phenol and levulinic acid, a platform chemical from renewable materials. Evidence is presented that the product with p,p’- and o,p’-isomers was obtained. Under the optimal conditions, the 93.2 mol% yield of DPA and close to 100% selectivity to p,p’-DPA were achieved in a process promoted by [BSMim]HSO4. The favorable results can be attributed to the special structures of ionic liquids and thiol compound involvement. Furthermore, a mechanism of condensation promoted by acidic ionic liquid with ethanethiol is proposed.