The catalytic systems composed of ionic liquids containing BF4- anion and HBF4 showed high catalytic activity to produce 4-methyl-2,4-diphenyl-1-pentene (MDP-1) or 1,1,3-trimethyl-3-phenylindan (TPI) under different temperature conditions. Up to 90.8% selectivity to MDP-1 with a 98.7% conversion of alpha-methylstyrene was obtained at 60 degrees C in the presence of [HexMIm]BF4-HBF4, while exclusive TPI was yielded when the reaction temperature increased to 120 degrees C. Further studies showed that another ionic liquid, [BMIm]Cl center dot 2AlCl(3), could act as an excellent catalyst and solvent for the dimerization of alpha-methylstyrene to produce TPI. The dimerization of alpha-methylstyrene catalyzed by [HexMIm]BF4-HBF4 and [BMIm]Cl center dot 2AlCl(3) performed the same reaction mechanism and the proton was the active species.