Catalytic pyrolysis of C4 hydrocarbons over a modified ZSM-5 zeolite was investigated in a fixed bed reactor. The effects of reaction temperature and dilution ratio on product yields and distribution were studied, ethylene yield goes up monotonously with the enhancement of reaction temperature; meanwhile, propylene yield shows a maximum at about 580 degrees C. As dilution ratio increases, the yield of propylene increases until a dilution ratio of 2.0 is reached, and then changes slightly, and that of ethylene passes through a maximum at about 2.0. For catalytic pyrolysis of n-butylene, two reaction pathways have been proposed, the monomolecular reaction mechanism and the bimolecular reaction mechanism. With the analysis of these two reaction pathways and the experimental data, a conclusion is drawn that catalytic pyrolysis of n-butylene follows the bimolecular reaction mechanism. According to this mechanism, n-butylene catalytic pyrolysis involves two successive reaction steps: the dimerization of n-butylene and the cracking of the dimers. (c) 2005 Elsevier B.V. All rights reserved.