Our previous work showed that for catalytic alkylation of m-cresol with tert-butanol (TBA) SO3H-functionalized ionic liquids exhibited several characteristic advantages over conventional catalysts. This work investigated the reaction mechanism of the alkylation of m-cresol with tert-butanol catalyzed by the SO3H-functionalized ionic liquid (IL) through quantum chemical calculation in combination with the experimental Studies. The experimental results showed that 2-tert-butyl-5-methyl phenol (2-TBC), 4-tert-butyl-3-methyl phenol (4-TBC) and tert-butyl-m-cresol ether (TBMCE) products were all primary products, while 2,6-di-tert-butyl-3-methyl phenol (2,6-DTBC) was a secondary product The calculation results indicated that the selectivities of the products depended on the fundamental natures of the reactive sites, including the orbital overlap, the Coulomb and the steric effect ill the interaction between the tert-butyl ion ([t-C4H9](+)) and the m-cresol: the TBMCE was dynamically favored but not thermodynamically stable, while the C-alkylated products, especially 2-TBC, were the thermodynamically preferred products: the IL played an important role ill generating the [t-C4H9](+) from the TBA and the final products from the intermediates. (C) 2009 Elsevier B.V. All rights reserved.