The dimerization of 1,3-cyclopentadiene has been studied in the chloroaluminate ionic liquids. The rate of dimerization of cyclopentadiene is higher in the 1-ethyl-3-methyl-1H-imidazolium chloride (EMIC)-containing chloroaluminate than in the N-1-butylpyridinium chloride (BPC)-containing chloroaluminate. Acceleration of the dimerization process is noted take place with increasing AlCl3 contents in both BPC and EMIC and is attributed to the Lewis acid catalysis. The temperature-dependent studies indicate that high temperature induces the dimerization. Quantitative information on the extent of dimerization of 1,3-cyclopentadiene in some Diels-Alder reactions has been offered in this work. (C) 2003 Elsevier B.V. All rights reserved.