Several SO3H-functionalized ionic liquids were used as the acid catalyst for the isopropylation of m-cresol with isopropyl alcohol. The results indicated that the sequence of the catalytic activity was in good agreement with the acidity order. A 77.7% conversion of m-cresol and 48.7% selectivity for 2-isopropyl-5-methylphenol were observed in the presence of [TMBSA][HSO4] (TMBSA = N,N,N-trimethyl ammonium-N-butanesulfonic acid). The catalysts could be recovered by extraction and reused, and after eight times of recycling, a loss of catalytic activity was not evident.