The adsorption and transformation of cyclopentadiene on HY and HZSM-5 zeolites were investigated by infrared (IR) spectroscopy and temperature-programmed desorption (TPD). The stoichiometric formation of monomeric cyclopentenyl carbenium ions (C5H7+) was observed on the acidic sites in the supercages of zeolite HY and in the channels of zeolite HZSM-5 at room temperature, without formation of oligomerized cyclopentadiene. The IR spectra indicate that addition of quantitative cyclopentadiene led to the stoichiometric consumption of acidic OH groups. These cyclopentenyl carbocations formed in the supercages affected the vibration of the remaining OH groups at both high and low frequencies, resulting in a shift of the OHHF from 3642 to 3530 cm(-1) as well as a shift of the OHLF from 3552 to 3500 cm(-1). The TPD-MS results reveal that the cyclopentadiene transformation on these H-form zeolites occurred at a temperature range of 473-800 K and followed a hydride ion-transfer pathway. The monomeric cyclopentenyl carbocation was the key intermediate initiating the cracking chain proceeding by the cationic mechanism. (c) 2008 Elsevier Inc. All rights reserved.