Dimerization of 2-phenylpropene using H-Y zeolite in the presence of diethylcarbonate (co-catalyst) in a liquid phase gave an industrially important linear dimer, 2,4-diphenyl-4-methyl-1-pentene (1-PT), as a major product. By the addition of diethylcarbonate, the formation of undesirable products, 2,4-diphenyl-4-methyl-2-pentene (2-PT), 1,1,3-trimethyl-3-phenylindan (IND), and trimers, was suppressed with a decrease in the dimerization rate. Neither Na-Y zeolite nor amorphous silica-alumina Al2O3-SiO2 showed any significant activity for the dimerization. Low reaction temperature favored the formation of 1-PT. The maximum yield of the target product, 1-PT, was obtained over the catalyst: 78% yield at 86% conversion under optimum conditions at 353 .